Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin Controls

Context Previous studies have reported that early initiation of cannabis (marijuana) use is a significant risk factor for other drug use and drug-related problems.

Objective To examine whether the association between early cannabis use and subsequent progression to use of other drugs and drug abuse/dependence persists after controlling for genetic and shared environmental influences.

Design Cross-sectional survey conducted in 1996-2000 among an Australian national volunteer sample of 311 young adult (median age, 30 years) monozygotic and dizygotic same-sex twin pairs discordant for early cannabis use (before age 17 years).

Main Outcome Measures Self-reported subsequent nonmedical use of prescription sedatives, hallucinogens, cocaine/other stimulants, and opioids; abuse or dependence on these drugs (including cannabis abuse/dependence); and alcohol dependence.

Results Individuals who used cannabis by age 17 years had odds of other drug use, alcohol dependence, and drug abuse/dependence that were 2.1 to 5.2 times higher than those of their co-twin, who did not use cannabis before age 17 years. Controlling for known risk factors (early-onset alcohol or tobacco use, parental conflict/separation, childhood sexual abuse, conduct disorder, major depression, and social anxiety) had only negligible effects on these results. These associations did not differ significantly between monozygotic and dizygotic twins.

Conclusions Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.

Over the past decade there has been a steady increase both in the prevalence of cannabis (marijuana) use among young people 1, 2 and in the number of people entering treatment for cannabis-related problems.3 In 1999 there were 220 000 cannabis-related admissions to publicly funded substance abuse treatment programs in the United States.³ This represented 14% of all such treatment admissions, with admissions occurring primarily among youth: approximately a third of all cannabis-related admissions were among people 12 to 17 years of age and a further third were among those 18 to 25 years of age. These increases in treatment seeking have been paralleled by heightened concerns about the long-term consequences of chronic cannabis use 4, 5 and a recognition of the need for treatment and other interventions to ameliorate the effects of drug dependence, which is best characterized as a chronic, recurring condition.6

The majority of cannabis-related admissions among youth result from referrals either from the justice or educational systems,7 and it is probable that at least some of these referrals were motivated more by concern over the future consequences of early initiation to cannabis use than by apparent negative effects of current cannabis use. A major focus of concern is the extent to which early cannabis use may increase the risks for escalation to other drug use and drug dependence. Stage theory posits that there is an invariant sequence in initiation and use of drugs, with use of cannabis preceding the use of "hard" drugs such as cocaine and heroin.8-10 This theory has been highly influential in drug policy debates and has provided a major rationale for sustaining prohibition against cannabis,11 as it is assumed that delaying or preventing early cannabis use may reduce risks of other illicit drug use.

While this broad theory has found some empirical support,8-10, 12-14 such data on temporal sequencing do not establish that the use of one drug causes the use of drugs higher up the sequence.^{11, 14} Rather, the observed pattern of initiation and use may reflect other factors such as availability and access.¹¹ Nonetheless, several studies using event history analysis 15 and regression analyses 16-19 have reported that early initiation to cannabis use remains a significant risk factor for both the use of other drugs and experiencing drug-related problems.

We are unaware of any studies that have controlled for genetic influences on the association between earlier initiation of cannabis use and other drug use and drug-related problems. Nonetheless, one viable hypothesis to explain this apparent association is that it arises from the joint influence of genetic and/or shared environmental factors on both risks of early initiation to cannabis use and, independently of this, on increased risks of subsequent drug use and dependence. This hypothesis is supported by evidence that cannabis and other drug use and drug abuse/dependence are moderately to highly heritable.20-22

The examination of other drug use and drug abuse/dependence in twin pairs discordant for early cannabis use provides a powerful test of the hypothesis that the association between early cannabis use and later outcomes can be explained by common predisposing genetic and/or shared environmental risk factors. Since these predisposing factors are shared by twin pairs raised together, if the association between early cannabis use and later drug use can be explained by shared environmental factors, then in twin pairs discordant for early cannabis use, individuals who do not initiate early cannabis use should be at equal risk of developing drug-related problems as their co-twin who initiates cannabis use early. If correlated genetic effects explain these associations, then monozygotic pairs discordant for early use should still have equal risks. In contrast, if the association is causal or explained by environmental factors for which twin pairs are discordant, we would expect to find higher rates of other drug use and abuse/dependence in the early cannabis user than in his or her co-twin. In this article, this issue is explored using data from a large community sample of young adult Australian monozygotic and dizygotic twins.

METHODS

Interviewees were members of the young adult cohort of the Australian Twin Register, a volunteer twin panel born between 1964-1971.20, 23, 24 The data presented in this report are derived from responses to a single telephone interview during the period 1996-2000 when the median age of the sample was 30 years (range, 24-36 years). Informed consent was obtained from participants prior to administering the interviews, as approved by the institutional review boards of Washington University School of Medicine and the Queensland Institute of Medical Research.

An overview of the study design is shown in Figure 1. Of 4010 pairs that could be traced, interviews were completed with both members of 2765 pairs (69% pair-wise response rate) and 1 member of another 735 pairs (78% individual response rate). A total of 861 members of the sample (13.7%) reported initiating cannabis use before age 17 years; 311 of these (36.1%) were from same-sex twin pairs in which their co-twin had not used cannabis by age 17 years. The analyses in this article are based on this subset of 622 same-sex twins from pairs discordant for early cannabis use. There were 74 female and 62 male monozygotic twin pairs and 84 female and 91 male same-sex dizygotic twin pairs. Zygosity was determined on the basis of responses to standard questions about physical similarity and confusion of the twins by parents, teachers, and strangers, methods that have been found to give better than 95% agreement with results of genotyping.25-28

Assessments

A structured diagnostic interview designed for genetic studies on alcoholism, the Semi-Structured Assessment for the Genetics of Alcoholism,29 was adapted for telephone use with an Australian sample and updated for Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) diagnostic criteria.30 The interview also included assessments of sociodemographic factors, childhood family environment, and experiencing childhood sexual abuse.²³ These measures are described below.

Measures of Subsequent Drug Involvement

Lifetime Drug Use
Respondents were asked whether they had ever engaged in nonmedical use of other drugs. In the analysis sample of 622 individuals from same-sex pairs discordant for early cannabis use, the following results were found: (1) sedative use (ie, benzodiazepines, barbiturates) was reported by 12.7% of women and 14.8% of men; (2) hallucinogen use was reported by 18.7% of women and 35.2% of men; (3) cocaine or other stimulant use was reported by 32.4% of women and 42.4% of men; and (4) opioid use was reported by 6.7% of women and 13.5% of men.

Lifetime Drug Abuse/Dependence
Individuals reporting using cannabis, sedatives, cocaine/other stimulants, or opioids on at least a monthly basis were asked additional questions concerning the extent to which they may have experienced symptoms of drug abuse (use in physically hazardous situations; use interfering with major role obligations) or dependence (using more frequently or for longer periods than intended; needing larger amounts to achieve an effect [tolerance]; continued use despite use causing emotional problems; recurrent desire to cut down on use). Abuse was operationalized by endorsement of either abuse symptom; dependence was operationalized by endorsement of 2 or more dependence symptoms. While the dependence measure did not provide formal DSM-IV criteria, previous analyses exploring the validity of these modified criteria for cannabis dependence indicated that they had both excellent sensitivity (96.7%) and specificity (94.6%) when compared with DSM-IV criteria.²⁰

Given the relatively low prevalence of drug abuse and dependence, measures of abuse and dependence for each drug class were combined. We assessed abuse/dependence for the following drug classes: (1) cannabis; (2) sedatives (ie, benzodiazepines, barbiturates); (3) cocaine/other stimulants; and (4) opioids. Additionally, these drug classes were combined to form a measure of any drug abuse or dependence. The prevalence of these outcomes is summarized separately by sex in Table 1.

Lifetime Alcohol Dependence
Lifetime alcohol dependence was assessed using full DSM-IV³⁰ criteria: 27.9% of women and 44.8% of men met criteria for alcohol dependence.

Family, Social, and Individual Factors

A number of family, social, and individual factors were included in the analysis as control variables. These were selected on the basis of a previous analysis with this sample that identified risk factors associated with cannabis dependence.²⁰

Psychiatric Disorders
Criteria for conduct disorder and major depression from the DSM-IV ³⁰ were assessed using the modified Semi-Structured Assessment for the Genetics of Alcoholism, and diagnoses were assigned by computer algorithm. A nondiagnostic measure of social anxiety was also defined.²³

Early Tobacco Use
A measure of early tobacco use was constructed by classifying subjects who reported smoking at least 1 day a week for a period of 3 weeks or more before age 17 years as early tobacco users (36.6% of twins from pairs discordant for early cannabis use reported such use).

Early Regular Alcohol Use
A measure of early alcohol use was constructed by classifying subjects who reported that they started drinking alcohol at least once a month for a period of 6 months or more before age 17 years as early regular alcohol drinkers (11.6% of twins from pairs discordant for early cannabis use reported such use).

Statistical Analyses

All statistical analyses were conducted using SAS 31 and STATA.32 As an initial test of heritability of onset of cannabis use, rates of concordance for early (before age 17 years) cannabis use were compared between monozygotic and dizygotic twin pairs. Differences in concordance rates were tested using the Breslow-Day test of heterogeneity of odds ratios (ORs), and separate tests were conducted for males and females. Conditional logistic regression models were then fitted to test for excess risk to early-onset cannabis users from same-sex discordant pairs, compared with their co-twin controls. The significance of the interactions between early cannabis use and both twin pair zygosity and sex were tested and, as these were nonsignificant (P>.10 in all cases), data were pooled across zygosity and sex. Analyses were repeated including the family and individual control variables described above. Stepwise regression with backward selection was conducted with the measure of early cannabis use forced into the model. These analyses were used to estimate conditional ORs for drug use and drug abuse/dependence in twins discordant for early cannabis use with control for other significant predictors.

Power was estimated using computer simulation. For example, for cocaine/stimulant abuse or dependence, we first obtained estimates of the prevalence (p_E = 12.5%, exposed twin; p_U = 4.5%, unexposed co-twin) and of the twin-pair tetrachoric correlation (r = 0.47). We then estimated the minimum OR = p_E(1 - p_E)/p_U(1 - p_U), such that a difference between the prevalence in the exposed twin and the unexposed co-twin would be detected with 80% power given a sample of 311 twin pairs at the .01 significance level, with p_U and r fixed at their observed values. Our results indicate that power would be 80% or better for an OR greater than 1.7 for 5 measures: hallucinogen use, cocaine/stimulant use, cannabis abuse/dependence, any abuse or dependence, and alcohol dependence. Power was over 80% for an OR greater than 2.5 for the measures sedative use, opioid use, and cocaine/stimulant abuse or dependence. Power was low under the reasonable range of OR for the measures sedative abuse/dependence and opioid abuse/dependence.

RESULTS

Rates of concordance for early cannabis use (before age 17 years) among the full interview sample (2765 pairs; see Figure 1) were significantly higher in monozygotic than dizygotic twin pairs for both men (65 concordant and 88 discordant monozygotic pairs vs 59 concordant and 110 discordant dizygotic pairs, chi

²₁ = 7.92, P = .005) and women (61 concordant and 98 discordant monozygotic pairs vs 44 concordant and 111 discordant dizygotic pairs, chi

²₁ = 7.80, P = .005), indicating heritable influences on age of initiation of cannabis use. The first 2 columns of Table 2 show estimates of the lifetime prevalence of drug use and drug abuse/dependence for those initiating cannabis use before age 17 years and for their co-twins (who either reported no lifetime cannabis use or who reported initiating cannabis use at age 17 years or older). The majority of subjects reporting both cannabis and other illicit drug use reported initiating cannabis use before initiating the use of other drugs. Three individuals reported initiating sedative use before cannabis use, 6 individuals initiated hallucinogen use, 5 initiated stimulant use, and 3 initiated opioid use before the use of cannabis. These individuals were excluded from the analyses. Table 2 also shows the conditional ORs, both unadjusted and adjusted for major risk factors, for the drug use outcomes. The results in Table 2 can be summarized as follows:

The final column of Table 2 shows the significant or marginally significant (P<.10) predictors of each outcome. While covariates differed between equations, early regular use of tobacco and alcohol emerged as the 2 factors most consistently associated with later illicit drug use and abuse/ dependence. While early regular alcohol use did not emerge as a significant independent predictor of alcohol dependence, this finding should be treated with considerable caution, as our study did not provide an optimal strategy for assessing the effects of early alcohol use.

Analyses Restricted to Those Reporting Lifetime Cannabis Use

Our analysis found that 24.8% of twins in pairs discordant for early cannabis use in fact reported no lifetime cannabis use. To examine the extent to which the associations in Table 2 may have been related to risks associated with any cannabis use rather than early cannabis use, these analyses were replicated with the sample restricted to those who reported lifetime cannabis use (54 monozygotic female pairs, 53 monozygotic male pairs, 56 dizygotic female pairs, 71 dizygotic male pairs). The results of these analyses, shown in Table 3, were broadly consistent with the previous results. In particular, before adjustment for covariates, early use remained a significant predictor of all but 1 of the outcomes (sedative abuse/dependence) among twin pairs concordant for lifetime cannabis use but where 1 twin reported initiation of cannabis use before age 17 years and the other did not. After statistical control for measured risk factors, early use was not a significant predictor of sedative abuse/dependence, and there was only a marginally significant association (P = .06) between early initiation of cannabis use and subsequent abuse/dependence on cannabis.

Subsidiary Analyses

Separate analyses of monozygotic- and dizygotic-discordant pairs were conducted in which early cannabis use was defined as use before age 15 years, before age 16 years, and before age 18 years. The results of these analyses confirmed the previous conclusions. For all definitions of early cannabis use, individuals who initiated cannabis use at an early age had elevated odds of later drug use and abuse/dependence relative to their discordant co-twin.

COMMENT

The results of our co-twin control analyses indicated that early initiation of cannabis use was associated with significantly increased risks for other drug use and abuse/dependence and were consistent with early cannabis use having a causal role as a risk factor for other drug use and for any drug abuse or dependence. Individuals who used cannabis before age 17 years had a 2.3- to 3.9-fold increase in odds of other drug use and a 1.6- to 6.0-fold increase in odds of alcohol dependence and other drug abuse/dependence, relative to their co-twin who had not used cannabis by age 17 years, regardless of whether or not the pair were monozygotic. Alternatively, there may be unmeasured environmental influences not shared by members of a twin pair that increase risks both of early cannabis use and of other drug use or abuse/dependence. We consider this less plausible, since twin pairs, having been reared in the same household, would be expected to be highly concordant for environmental experiences. Unmeasured family background risk factors or heritable risk factors cannot explain the observed association, since twin pairs will share the same family background, and monozygotic pairs the same genetic risk factors.

Potential limitations of this study include the reliance on self-report and retrospective data, and the lack of data about ages at progression to more frequent use or onset of problems. Age of first use was obtained earlier in our interview than the assessment of drug use problems to minimize recall biases; and the associations with early cannabis use that we observed in this twin pair sample have previously been reported in prospective studies.^16-19 An association due to underreporting of drug use by 1 twin seems implausible, since cannabis use, even by self-report, was highly prevalent and some use at least would be considered normative for this birth cohort in Australia, and since significant associations remained when analyses were limited to pairs concordant for lifetime cannabis use. It is also unlikely that we are observing only delayed onset of other drug use or drug abuse/dependence, rather than lower lifetime rates in the co-twins, since the median age of the sample (30 years) is considerably higher than typical ages of onset of drug use and drug abuse/dependence.33 For example, more than 95% of cannabis users in our total sample reported onset of marijuana use by age 24 years, the youngest age represented in the sample. Our estimates of the lifetime use of cannabis and other drugs are high but are consistent with those reported by other large-scale epidemiological surveys of drug use in Australia.34 Further, a recent study of cannabis use in a US national twin sample concluded that twin studies of substance use are unlikely to be biased and that findings from such studies can be generalized to other (nontwin) family relationships.35 The relative crudeness of our measure of exposure (any use before age 17 years as opposed to frequent, heavy, or problem use) makes the findings of an association even more remarkable.

If the association with early cannabis use is indeed causal, the mechanisms by which this association arises remain unclear. Pharmacological mechanisms might be hypothesized in which it is assumed that exposure to cannabis induces subtle biochemical changes that encourage drug-taking behavior.36 This hypothesis is supported to some extent by recent findings that Delta

⁹-tetrahydrocannabinol and heroin have similar effects on dopamine transmission through a common µ₁opioid receptor mechanism 37 and that chronic treatment with Delta

⁹-tetrahydrocannabinol induces cross-tolerance to amphetamine 38 and opioids 38, 39 in rats. However, an argument against such biological hypotheses is that the levels of cannabis use at the beginning of drug-using careers are substantially lower than the equivalents used in laboratory-based research and perhaps too low to induce long-term biochemical changes.

Other mechanisms that might mediate a causal association between early cannabis use and subsequent drug use and drug abuse/dependence include the following:

While the findings of this study indicate that early cannabis use is associated with increased risks of progression to other illicit drug use and drug abuse/dependence, it is not possible to draw strong causal conclusions solely on the basis of the associations shown in this study. Further research in other cultures and using a range of innovative research designs (including evaluation of prevention efforts aimed at delaying the onset of cannabis use) is needed to explore whether there is a causal link between early cannabis use and progression to other drug use and, if so, to elucidate the mechanisms that may underlie any such causal association.

Regardless of the mechanisms underlying these associations, it is apparent that young people who initiate cannabis use at an early age are at heightened risk for progressing to other drug use and drug abuse/dependence. In addition to cannabis dependence, the health risks associated with chronic cannabis use may include chronic bronchitis, impaired lung function, and increased risks of cancers of the aerodigestive tract.⁴ Given historical increases in the use of cannabis and other drugs,^{1, 2} it is probable that more individuals will experience these adverse consequences and there will be an increasing need to develop strategies both to prevent and to ameliorate the adverse consequences of chronic drug use. Given that early initiation of use may be associated with increased risks both for progressing to the use of other drugs and for developing drug abuse/dependence, there is a need for greater physician awareness of those risks associated with early use. There is also a need to develop focused interventions to prevent escalation to use of other drugs among young people identified as being at risk due to their early initiation of cannabis use.

Author Affiliations: Queensland Institute of Medical Research, Brisbane, Queensland, Australia (Drs Lynskey and Martin and Ms Statham); Missouri Alcoholism Research Center and Department of Psychiatry, Washington University School of Medicine, St Louis (Drs Lynskey, Heath, Bucholz, Madden, and Nelson); and Missouri Alcoholism Research Center and Department of Psychology, University of Missouri, Columbia (Dr Slutske).

Corresponding Author and Reprints: Michael T. Lynskey, PhD, Missouri Alcoholism Research Center, Department of Psychiatry, Washington University School of Medicine, 40 N Kingshighway, Suite One, St Louis, MO 63108 (e-mail: mlynskey@matlock.wustl.edu).

Critical revision of the manuscript for important intellectual content: Bucholz, Slutske, Madden, Nelson, Statham, Martin.

Administrative, technical, or material support: Heath, Slutske, Statham, Martin.

Funding/Support: This work was supported by National Institutes of Health grants AA07728, AA09022, AA10249, AA11998 (Dr Heath), AA 12640, DA 14363, DA 14632 (Dr Bucholz), DA00272, DA12854 (Dr Madden), and AA00277 (Dr Nelson), as well as grants 951023 and 981351 from the National Health and Medical Research Council (Dr Martin).

Acknowledgment: We thank the Australian Twin Registry and the twins themselves for participating in this research.

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While covariates differed between equations, early regular use of tobacco and alcohol emerged as the 2 factors most consistently associated with later illicit drug use and abuse dependence.

Then why are they hyping cannabis?

Just off the top:
1. Telephone interviews? Ohhhh, no chance for investigator bias there, huh?
2. 69% responding? 31% not responding? Well, better than an internet "survey", I suppose, but there might just be a bit of self-selection there, huh?
3. Causal relationship? Absolutely not shown. Here again, even if this sham were a prospective, double-blind study, the cause is not shown. There may be correlation, but one cannot infer cause.

Our long prior knowledge is reinforced about two things:

1. Psychologists who do research will always come up with the inference that satisfies their employers, i.e., governments who supply grant money.

2. JAMA is a political, quasi-medical, organization who should be ashamed to publish this.

So, what they seemed to have proved is that people who use one drug are quite likely to use another. I don't see where they've proved that use of one particular drug in turn leads to the use of others. Seems to me that you could just as easily, and probably more accurately, sum up the study as "People who are prone to use drugs find marijuana most readily available and inexpensive, and its use easiest to conceal." God knows my 16-year old son would find it a lot easier to buy and use marijuana than tobacco or alcohol.

I don't see anything here to tell me that use of marijuana led to the use of other drugs, and that if the person involved had not used marijuana, they wouldn't have picked up some other drug later on.

"The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs."

Translation: When the subjects learned that they had been lied to about the harmful effects of marijuana and that they did not in fact turn into raping murdering lunatics, they concluded that maybe they had been lied to about other drugs and that it would be ok to try them also.

"The JAMA's report in full, on the twins study, FYI."

Political science as usual. Wonder if these guys have ever worked for the United States Fish and Wildlife Service or the National Marine Fisheries Service?

The other glaring error in design study (which they sort of cop to in the quote in my previous post) is that nowhere do they control for what came first. I find it hard to believe that these people didn't try cigarettes and alcohol before marijuana. My guess is this "study" was put together as pure propaganda to counter the RAND study of a few months ago.

DAY of SUPPORT....FLY your flags (US, a British one, Hungarian, Australian and Japanese one, too if you have them)....and put up your BUSH/CHENEY signs, (and the BIG W's on your SUV's) for the STATE of the UNION next Tuesday, Jan 28th, if you support the President, our MILITARY and the United States of America. PSST....pass it on.

Then why are they hyping cannabis?

Because alcohol and tobacco producers have already won their battles in the War Against Some Drugs. Big business, don't you know. And they've also proven that Prohibition of their products doesn't work. Whereas Prohibition of cannabis doesn't have any deleterious effects (;P)

Its pretty funny though:

"Marijuana, Weed, Cannabis, Dope, Weed, Marijuana,...oh yeah, tobacco and alcohol are the greatest factors in association with later problems, now back to our regularly scheduled program...Marijuana, Weed, Grass, Dope..."

Devastating? Exactly how so?

Conclusions Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.

I agree it's not as fun as cheering on a cop for gunning down a dog and suggesting everyone "move on," but this is lazy for you, Kevin.

I don't have time to read through the whole thing right now. However, the conclusion:

Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.

is (while not ridiculous) at least overstated, based on first principles. We can't say why people choose marijuana in the first place, they may be inherently greater risk takers, experimenters or whatever. No two people grow up in the same environment, or are biologically identical. Some twins I've known would actually make deliberately different choices in life, in order to show independence from each other. Some people may simply be predisposed to using drugs, and no amount of correcting for other variables is going to capture that fact.

However, this is still the best approach to study the problem. Thanks for the flag.

"The other glaring error in design study" Well the whole gateway theory was found to be invalid years ago but I went ahead and started to read some of this as it does present information in some type of factual format. However, after just a couple of page down's it was obvious that it was riddled with so many assumptions and doubletalk as to be meaningless.

I did find the following line interesting.

" Stage theory posits that there is an invariant sequence in initiation and use of drugs, with use of cannabis preceding the use of "hard" drugs such as cocaine and heroin.8-10 This theory has been highly influential in drug policy debates and has provided a major rationale for sustaining prohibition against cannabis,11 as it is assumed that delaying or preventing early cannabis use may reduce risks of other illicit drug use.

So in the one part they are telling you that it is imperative that the gateway theory be validated as that is a key component in supporting the whole prohibition against marijuana, they then go on to tell you in the next part of the sentence that the whole gateway theory is assumed.

If I read it correctly, they did identify some individuals who had used some other classes of drugs prior to trying marijuana, and excluded them from the test. They don't seem to have made any attempt to determine if alcohol was used prior to trying marijuana. Curiously, they consider the tendency for an individual to become alcohol dependent after early exposure to marijuana to be of significance.

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