Is cannabis; also called marijuana, 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%.
Of course, the existence of a positive correlation between cannabis use and psychosis does not allow us to draw a direct causal link between the two. Psychosis could cause cannabis use, rather than vice versa, or an unknown mediating factor could cause both cannabis use and psychosis.
Other factors already complicate the relationship. For example, the age at which cannabis use begins appears to be an important factor. People who started using cannabis before the age of 16 have a higher risk of psychosis compared to those who start after 18. This is in line with biological evidence suggesting that the maturing brain is more susceptible to the negative impact of cannabis.
Recent research has shown that a particular variant of a gene called AKT1 mediates the risk of psychosis. For carriers of the C/C variant (which occurs in roughly 20% of the population), the risk of psychosis increased seven-fold - but only for the people who used cannabis every day. Using cannabis on weekends or less often posed no increased risk, which was also the case for daily and weekend use among carriers of the C/T and T/T variants of AKT1.
Moreover, different strains of cannabis pose more or less risk for psychosis. A comparison of the two most common types of cannabis in the United Kingdom, hash and sinsemilla ("skunk"), found that using skunk implied a significant greater risk of psychosis, while hash did not.
The explanation for this difference lies in the composition of the two main components of cannabis, delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). Skunk in the UK has high levels of THC while being virtually devoid of CBD; hash, on the other hand, has roughly equal quantities of the two components. This was highlighted in our lab when healthy volunteers were given either pure THC or THC and CBD.
Those who received only THC had a significant increase in paranoid thoughts, psychotic symptoms, and memory impairment, while those who received the combination experienced no paranoia, had fewer psychotic symptoms, and maintained memory function.
Some studies have also explored the role of cannabis use in disorders such as depression and anxiety. While little sign of a link has been found, there is better evidence for the addictive potential of cannabis. Roughly 10% of people who smoke cannabis go on to develop dependence, which produces withdrawal symptoms when use is stopped, such as craving, irritability, sleep disturbances, abdominal pain, and nervousness. Again, CBD seems to offset the addictive effects of cannabis, and seems to provide relief from withdrawal symptoms.
Many studies have focused on the possible negative impact of cannabis use on memory and other cognitive functions. It is generally accepted that frequent and prolonged cannabis use impairs cognitive functioning, but that these effects are reversible following abstinence of 3-12 months. However, a recent study that followed people from birth to age 38 found that those who started using cannabis early, every day, and for several years had a permanent 8-point drop in IQ scores. (The study has not yet been replicated.)
A final, and often-overlooked, source of harm to mental health associated with cannabis is the legal fallout of being caught with it. Penalties for cannabis possession range from none (for example, the Netherlands and Portugal) to legal warnings, fines, and even life imprisonment (Southeast Asia).
A criminal record can negatively affect future employment, education, and ability to travel. Such consequences may carry independent risks to mental health; for example, unemployment poses a significant risk for suicide, anxiety, depression, and psychosis.
So, does cannabis use harm mental health? Yes and no. Much depends on how old you are when you start, your genes, and how much, how often, and what kind you use - and, of course, on whether you get caught.
Amir Englund is a researcher at King's College London. Robin MacGregor Murray is Professor of Psychiatric Research at the Institute of Psychiatry, King's College London, and a fellow of the Royal Society.