The Lancet Psychiatry has published a study which finds that daily users of cannabis are three times more likely to develop schizophrenia than those who never use it. The cannabis legalisation lobby will no doubt be picking holes in the research, and I hope they do a better job than Suzi Gage in The Guardian who resorts to making generic criticisms of epidemiology and saying 'correlation isn't causation' (as she tends to do when she doesn't like the findings but can't spot a specific flaw).
I am a card-carrying member of the cannabis legalisation lobby myself, but I don't need to pretend that cannabis is without risk to defend my position. I can do so on moral and practical grounds. The moral (libertarian) argument is that it is not for the government to persecute people for taking drugs so long as users are reasonably well-informed about the risks. The pragmatic (free market) argument is that the social and economic costs of prohibition are greater than costs of regulated drug use.
My reading of the evidence is that cannabis probably is associated with psychosis and schizophrenia in some way and that questions about reverse causation and confounding have been largely answered (see here, here, here and here). The new Lancet study does not come as a surprise, but it is made more interesting by its analysis of what type of cannabis is associated with psychosis.
These are the main findings:
Three points stand out. Firstly, that there is no association between cannabis use and psychosis for those who started smoking the drug after the age of 15. Secondly, that only those who use cannabis very frequently are at higher risk. Thirdly, that it is only skunk marijuana—which is high in THC—that seems to increases risk.
The difference between hash and skunk consumption in the case and control groups is striking. 44 per cent of the control group had smoked hash, but only 14 per cent of the group with psychosis had done so, whereas only 19 per cent of the control group had smoked skunk while 53 per cent of the group with psychosis had done so.
Opponents of drug reform will use this study to support continued prohibition—the Home Office has already done so—but it is actually another reason to liberalise.
The purpose of legalisation is not to have a free-for-all but to have a regulated market. In my book The Art of Suppression, I set out a proposal for legalisation that would allow drugs to be sold in much the same way as alcohol and tobacco. It is a blueprint that paves the way for opium bars, head shops and the sale of regulated drugs in pharmacies and nightclubs, for example, but it does not see a place in the market for every drug that has emerged in the century of prohibition.
Alcohol is legal, but you cannot go into an off licence and buy a litre of pure ethanol. Similarly, a sound system of drug legalisation would allow the sale of opium, but not heroin. It would allow the sale of cocaine (or cocaine-based products), but not crack. And it would allow the sale of pre-rolled, manufactured spliffs but there would be limits on THC content that would effectively ban the sale of skunk.
Would this system eliminate demand for crack, heroin and skunk? Probably not—and certainly not overnight—but it would help to shift consumers away from the strongest and more harmful derivatives of drugs. Who bought moonshine after Prohibition was repealed?
The aim is to bring the market back to where it was before the war on drugs began—when consumers, not suppliers, called the shots. Prohibition shifts power from consumers to suppliers. The sale of strong spirits soared during Prohibition while beer sales fell. Why? Because beer was too bulky for bootleggers and spirits offered greater profits. Skunk, crack and heroin offer the same benefits to drug dealers. Being highly concentrated, they are less bulky and therefore easier to smuggle and carry. The strongest derivatives always prosper under prohibition, but they carry the greatest health risks.
So it is with cannabis. Skunk was developed under prohibition for prohibition. It cannot now be uninvented, but a system which allowed the general sale of lower-THC varieties (to adults) would undo much of the damage that prohibition has done in this, as in every other, market.