Cannabis use is considered a contributory cause of schizophrenia and psychotic illness. However, only a small proportion of cannabis users develop psychosis. This can partly be explained by the amount and duration of the consumption of cannabis and by its strength, but also by the age at which individuals are first exposed to cannabis. Genetic factors, in particular, are likely to play a role in the short- and the long-term effects cannabis may have on psychosis outcome. … Evidence suggests that mechanisms of gene-environment interaction are likely to underlie the association between cannabis and psychosis.
Obviously, only a fraction of pot smokers are going to go crazy and join the 1-3% of Americans who are psychotic. Think of smoking marijuana on a regular basis as playing Russian roulette once with a 50-shot cylinder, one of which has a live round. (Of course, now that you know that, maybe you do have to be crazy to smoke marijuana.)
At this point, you may be saying: “Big deal! It’s my life! If I want to smoke pot and risk going crazy, that’s my choice!” I would concede that point, except that as of 2002, schizophrenia alone of the mental disorders was costing the United States $63 billion a year in medical costs and in disability payments. Much of that cost is directly governmental, since schizophrenics usually aren’t able to work and thus are reliant on the government.
You might also argue: “What about alcohol? Doesn’t it have risks?” No question — and these risks have been recognized for a long time. Arguing for decriminalization of marijuana because alcohol is a big problem is like arguing that because one of your feet is gangrenous the doctor should also amputate the healthy foot just to be even-handed. (Or even-footed, I suppose.) If anything, instead of decriminalizing marijuana, we should be looking at discouraging alcohol — and recognizing that while Prohibition didn’t work, there may be approaches more educational, and less drastic, that can.