Via The Drug Update, I see that someone’s trying to drag out the old gateway theory again. The gateway theory really only holds true in one respect — that people who buy marijuana have to do so from criminals who also sell other drugs, so they may be convinced to buy those as well (an argument against prohibition).
Yet people continue to try to promote or “prove” the biological effect, or “stepping stone” version of the gateway theory. The latest is at under the fancy title: Adolescent Cannabis Exposure Alters Opiate Intake and Opioid Limbic Neuronal Populations in Adult Rats
Here’s what they claim to have discovered:

THC-pretreated rats showed an upward shift throughout the heroin self-administration acquisition… phase, whereas control animals maintained the same pattern once stable intake was obtained.

In other words, if you happen to be a THC-pretreated rat (and I’m sure there are some of you out there reading this), and you decide to use heroin, you may have the desire to increase your heroin use. If, however, you are not a THC-pretreated rat, but rather an ordinary rat, then your heroin use will probably be stable.
Their conclusion:

The current findings support the gateway hypothesis demonstrating that adolescence cannabis exposure has an enduring impact on hedonic processing resulting in enhanced opiate intake, possibly as a consequence of alterations in limbic opioid neuronal populations.

Actually, no. The gateway hypothesis (as it is popularly used by the media and politicians) has to do with an increased predilection for both the use and abuse of the latest horror drug-du-jour, ie., “marijuana will lead you to heroin addiction,” not “marijuana use when you’re young will, if you use heroin when you’re older, make it harder to quit” (if you’re a rat).
The use of the word gateway in the abstract of this study is a blatant attempt to get publicity for their study, likely with the full knowledge that their data will be misused.
Pretty poor science, considering they really don’t have much of a clue what it is that they’ve learned.
There’s one very clear rebuttal to the gateway theory. If marijuana use is dangerous because it leads to heroin addiction (as the popular theory goes), then all we need do is look at those who have tried marijuana (96.8 million Americans) and then see how many are currently addicted to heroin (No reliable figures exist for this, so using the same data standards (White House drug facts), we’ll simply go with reported past month use: 166,000. Of course, there’s no way of knowing how many of these are actually addicted, but we’ll use it anyway.)
This means that at least 99.83 percent of those who have tried marijuana have not gone on to become heroin addicts today.
Peculiar sort of gateway, where less than 2/10ths of one percent manage to find their way through it.
Update: Maia Szalavitz also finds it to be agenda-based science reporting.

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