In a previous post I argued that Humanists should be concerned about the international prohibition of recreational drugs. I cited evidence to show that the 'war on drugs' has been lost and that much of the harm associated with drug use is due to illegality not pharmacology. I did not make the libertarian case; though humanists ought to take it seriously.
I did not and never have claimed that the use of street drugs was risk-free. I've always believed that heroin, for instance, is very harmful to most of its users. But cannabis is surely different. When I first became interested in this topic - 40 years ago - the evidence against cannabis seemed very slight - and generally contradictory. Yet I've had to accept from more recent evidence that cannabis use is associated with an increased risk of psychosis.
So I was wrong? Sort of. But the truth is more complex.
Cannabis has several active ingredients notably tetrahydrocannabinol (THC) and cannabidiol (CBD). Laboratory and field studies show that it's THC that creates the risk of psychosis whilst CBD has a protective effect.
Over the years the 'hash' and 'grass' of my youth have been replaced by 'skunk'. Skunk contains a higher concentration of THC than the hash of yesteryear and, critically, much less CBD. It's therefore much more likely to provoke psychosis.
This change in composition is not some random effect of climate or fashion; it's a consequence of prohibition. Over the years street cannabis supply has switched from imports from high hills in hot countries to domestic cultivation under intensive lighting. The intensive lighting appears to drive the replacement of CBD by THC.
This would probably not have happened without prohibition and if it had happened in a regulated market then the regulator could have intervened to reverse the trend.
The current incidence of cannabis-related psychosis is therefore a consequence of prohibition. There would be some cannabis-related psychosis in a regulated legal market - cannabis is not risk-free - but if we care about health we will abandon prohibition and seek a policy that might reduce the harm from drug use.