LONDON – Is cannabis harmful to mental health? That question has provoked much debate over the years, often generating more heat than light. The bottom line is this: within the scientific community, there is a general consensus that most people who use cannabis will not develop significant mental-health problems. Certain individuals, however, are more susceptible to the negative effects of its use.
It was long thought that cannabis was a relatively harmless drug, and that concerns about its use were overstated. Some psychiatrists had reported that excessive use could lead to a psychotic state, including hallucinations, delusions, and thought disturbance. But the first significant indication of a link between cannabis use and psychotic illness came only in 1987, from a large Swedish study that followed more than 50,000 subjects over 15 years. Reported cannabis use at the study’s start increased the likelihood of being diagnosed with schizophrenia in the next 15 years. The greater the use, the higher the likelihood of being diagnosed.
Curiously, this finding did not spur much interest, and no similar studies were reported until 2002. Since then, however, many studies have explored the association between cannabis use and psychotic illness. In 2007, a compilation of the best studies concluded that frequent (daily) cannabis use doubles the risk of a psychotic outcome. Since the lifetime prevalence of a psychotic illness is roughly 1% of the population, daily cannabis use would increase this to 2%.