And yet this study was trumpeted as proof of the gateway theory. It proved no such thing, as the heretofore un-reported details show.
Maybe by the press it was. But the study itself attempted no such thing. The actual title of the study was, "Adolescent Cannabis Exposure Alters Opiate Intake and Opioid Limbic Neuronal Populations in Adult Rats".
The Abstract of the study states, "(Since) Cannabis use is a hypothesized gateway to subsequent abuse of other drugs such as heroin ... (we) ... assessed whether -9-tetrahydrocannabinol (THC) exposure during adolescence modulates opiate reinforcement and opioid neural systems in adulthood." They gave this a their reason for using cannabis instead some other drug.
As to the "gateway effect", the authors of this study referred to four other studies that supported that:
"Several epidemiological studies report that early regular use of cannabis increases the risk of initiation of the use of other illicit drugs (Agrawal et al, 2004; Fergusson and Horwood, 2000; Lynskey et al, 2003; Yamaguchi and Kandel, 1984), supporting the cannabis gateway hypothesis of cannabis as a steppingstone toward abuse of other drugs."