MarkH over at the denialism blog justifiably dissects a mainstream media article sensationalizing a study on the effects of marijuana smoking on lung function. He then goes on to express drug-legalization denialist (see this comment as well) positions as a rebuttal to the drug-risk overstatements. To this I take exception. Most specifically the use of mainstream media hype and/or the usual scientific authors’ hyperbole making more of a study than is perhaps deserved to attempt to question a reasonable interpretation of the available scientific evidence. Also, I find a minimization technique suggesting that since the effects of marijuana withdrawal do not look like nicotine withdrawal, marijuana must only be “psychologically” addicting, not “physiologically” addicting particularly annoying. I address myself to this relatively common argument, as expressed in this instance by MarkH.
First, what in the heck is the basis for the use of “psychologically” and “physiologically” addicting? Are you a dualist who maintains that there is a “mind” or “psyche” that exists independently of the workings of the brain? I’m not. So by definition, if something is altering behavior, it is “physiological”. If one is attempting to dissociate somatic from brain symptoms, well, good luck with that. Yes there are some dissociable things but the interplay is really too involved, and the ultimate response that matter, i.e. behavior, is primarily brain in nature. Or are you one of these people that believes that many (all?) mental/behavioral disorders are due to personal choice and lack of “willpower” and that people should just snap out of it? This is what people usually mean when they say something is “psychological” and a contrast with “physiological” certainly suggests this is what one means. This is just as stupid for drug dependence as it is for major depression, ADHD and the like. MarkH commented that “physiological addiction has not been firmly established”. Really? How so? Take a little spin on PubMed for “cannabis withdrawal” or similar. The only way you can come up with an interpretation like Mark’s is by highly selective reading and denial of the literature in my view.

Second, beware of goalpost juggling related to this issue. Is your frame of reference for “addiction” is the rather dramatic phenomena of heroin withdrawal (perhaps a movie cartoon fictionalized version? not ALL heroin addicts look exactly alike in withdrawal severity) and presumably direct experience with nicotine withdrawal? The apparent severity, drama or somatic-pain-like nature of withdrawal from different drugs of abuse is not an exclusive indication of whether or not they are “physiologically addicting”. For that matter it is not even relevant to a reasonable discussion of “how addicting” because for this latter you need the behavior, i.e., continued drug taking in the context of a desire to quit, adverse consequences, etc.

Third, THC is a reasonably unique drug of abuse because it has a long halflife. Take a simplified version, say where withdrawal sets in once brain levels of drug / receptor occupancy decline. Symptoms can be alleviated by replacing this with a less effective but similarly acting drug like methadone for heroin or a limited, sustained dose like the nicotine patch (this is agonist therapy). The long halflife of THC in the body means that you have ongoing agonist therapy (low levels of drug still present, say, the next day) through an interval in which withdrawal would be observed after discontinuing nicotine or heroin. Again, the point is not the relative severity of symptoms or ecological significance for the organism. The point is that a subjective understanding of whether or not the human is “physiologically addicted” is compromised without accounting for this difference in pharmacokinetics. This time, try using “precipitated” in your search to review experimental ways around this issue.

Fourth, the “my friend Joe” argument (“My friend Joe smoked dope all day every day for 10 years and then just quit one day with no lasting symptoms” or similar), common to many legalize-it types, is just as flawed as the “one toke/injection and you are hooked for life” ReeferMadness type position. Donny and Dierker 2007 report that almost 40% of daily smokers (at least 10 / day for at least 10 years) don’t reach DSM criteria for dependence! Anthony et al 1994 (Exp Clin Psychopharm) shows us that the conditional probability for meeting dependence criteria given that you have experienced a drug at least once. Not the only way to calculate such a thing of course. But it suggests that only about 8% of cannabis smokers are dependent and, wait for it, only about 24% of heroin injectors are dependent. The relative population prevalences are much greater for cannabis (46% versus 1.5% in that sample), thus many, many more people are dependent on cannabis than on heroin. Odds are a given person knows a lot of cannabis users and next to zero heroin users. Even if one does know a lot of heroin users, the chances that person will be dependent is much greater. Finally, because most people’s assessment of “dependence” is biased for the drama as discussed above, a subjective impression of dependence is wildly off base.

Ultimately what I am trying to get at here is that the legalize-it crowd is just as prone to misusing science as is the bogey-man of the great Gov/DrugCzar/NIDA/Republican conspiracy to keep dope illegal. Reefer madness approaches are nutso. So are people that feel that chronic THC exposure is perfectly benign. So are people that believe that THC is not addicting. As someone who spends a deal of time trying to parse the actual risks of drugs of abuse scientifically, both types of misuse / ignorance of the available science are distasteful.