Whereas alcohol makes it better?
(1) Opposition to marijuana legalization is not an endorsement of alcohol, nor is it a recommendation that schizophrenics take up drinking.
Beside the point, of course. A case for legal alcohol and illegal marijuana falls flat if it cites harms of the latter that are shared by the former.
(2) A serious argument for marijuana legalization ought to demonstrate that the harms of marijuana use will not be amplified by its legalization. That seems implausible though.
A serious argument for marijuana remaining illegal and alcohol legal ought to demonstrate that the harms of marijuana use will be amplified by its legalization more than the harms of alcohol are. That seems implausible though.
A serious argument for marijuana legalization need only point out that, like alcohol criminalization before it, the primary effect of marijuana criminalization is to enrich criminals with all the harms to innocent third parties that this entails.
(3) I have never smoked marijuana, but, like most people these days, I have seen enough of its ill effects to incline me to oppose legalization.
A serious argument for marijuana remaining illegal and alcohol legal is required not only to establish that marijuana is harmful - which, I remind you again, nobody here has disputed - but that its harms exceed those of alcohol. Surely you've seen the ill effects of alcohol?
Unless a case can be made based on first principles, the case for marijuana legalization should fail on pragmatic grounds if it can be shown that legalization and the resulting increased access and use would impose significant new harms and risks on society. The medical evidence indicates that to be the case in the form of the mental illness associated with marijuana use.
For example, in 2013, in Association between cannabis use, psychosis, and schizotypal personality disorder: findings from the National Epidemiologic Survey of Alcohol and Related Conditions, the authors of a major study reported that:
The results indicate that the risk of both psychosis and SPD [schizotypal personality disorder] increases with greater use of cannabis, in a dose-dependent manner. Compared to non-users, greater cannabis use showed significantly elevated risk of having been diagnosed with SPIE [self-reported history of psychotic illness or episode] and elevated risk of all SPD symptoms, even after adjusting for sociodemographic characteristics.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108251/
Similarly, in 2014, in Prognosis of schizophrenia in persons with and without a history of cannabis use, the authors reported that:
In addition to our previous findings of an increased risk of schizophrenia in subjects with history of cannabis use, we now show that schizophrenia patients with a history of cannabis use also have a poorer prognosis, as indicated by longer hospital episodes and more readmissions. Thus, it is of public health as well as clinical importance that, as well as increasing risk of schizophrenia, cannabis may also lead to an illness that is more severe than in non-users of this drug.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4108251/
I note that although marijuana use is apparently responsible for a greater risk of and severity of mental illness, that does not commonly amount to a certainty or probability of mental illness in otherwise healthy individuals. The cumulative effect for society though is for increased marijuana use to impose an increased burden of mental illness.
Do not take my word for any of this. Read my sources and do your own review of the medical evidence. I submit though that, unless you can refute my demonstration with evidence of a similar quality, I have won the point: marijuana legalization must logically be expected to lead to more cases of schizophrenia and an increase in their severity and burden. Within a few years, the experience of the states that have legalized marijuana will offer more definitive proof on this point.