Can people overdose on cannabis?

Not easily. Scientists estimate that the lethal dose of THC, the active ingredient of the drug, is somewhere in the range of 15-70g. That is far beyond the daily consumption of even the most enthusiastic user.

Cannabis: scientists call for action amid mental health concerns Read more

What are the short-term effects?

Beyond the high that attracts about 180 million people a year worldwide, side effects range from anxiety and paranoia to problems with attention, memory and coordination. The acute mental impairment that comes from cannabis smoking explains why stoned drivers are twice as likely to crash their cars than unimpaired drivers.



Can people become dependent on cannabis?

Yes. Studies suggest that one in 10 regular cannabis users become dependent on the drug, or one in six of those who start in their mid-teens. The number of people seeking professional help to quit or control their cannabis habit has risen in the past 20 years in Europe, the US and Australia. Only alcohol and tobacco lead more people into treatment. People in withdrawal can experience anxiety, insomnia, depression and a disrupted appetite.



Facebook Twitter Pinterest Marijuana smoke-in protest at the White House In Washington. Photograph: UPI/Barcroft Media

What are the long-term effects?

Mental health problems are one of the greatest concerns. Nearly 30 years ago, a study of Swedish conscripts found that those who reported using cannabis more than 50 times by the age of 18 were three times more likely than others to have schizophrenia at 45. Other studies support the findings but all have weaknesses: they do not prove beyond doubt that cannabis causes mental health problems.



Is high-potency cannabis more harmful than weaker strains?

Many researchers think so. Last year, scientists at King’s College London studied a population of south Londoners and found that those who smoked skunk every day had five times the normal risk of psychosis. A later study from the same institution found that those who smoked skunk daily had subtle changes in their white matter that could impair communication between the two sides of the brain. The changes were not evident in non-smokers, or those who smoked low-strength cannabis.



How big is the risk to regular users?

Daily users have a 2% chance of developing schizophrenia in their lives, about double that of the general population, though the risk of other less serious mental health issues is greater. Efforts are now underway to work out who are the most vulnerable people. In a review published this month, Bristol University scientists say the evidence is now strong enough to warrant public health messages that cannabis can increase psychotic disorders.

What is in cannabis?

Cannabis plants produce more than 100 active compounds called cannabinoids. The one most responsible for the mind-altering effects of the drug is called delta-9 tetrahydrocannabinol, or THC. Growers crossbreed plants to produce high-THC strains to give them more bang for the buck. The drug has its most immediate effects when inhaled. Within seconds, THC in the smoke passes into the bloodstream and then into the brain. A dose of 2-3mg of THC is enough to produce a high.

How does it work?

Inside the brain, THC latches on to what are called cannabinoid receptors. These are found in huge numbers in the hippocampus, where THC affects memory; the cerebellum, where it affects coordination; and in the basal ganglia, where THC can also affect movement.



Does THC mimic chemicals made in the body?

Yes. The body produces its own THC-like substances called endocannabinoids. These are released in the body to help regulate a host of different functions in the brain. Mood, cognitive function and memory are all affected by THC and the body’s endocannabinoids.

Facebook Twitter Pinterest A demonstrator showing cannabis during a protest in front of parliament in London. Photograph: Gail-Orenstein/Demotix/Corbis

How might THC be harmful?

Frequent high doses of THC may overstimulate the endocannabinoid system. “Too much activity usually leads to what we call down-regulation of the system,” said Amir Englund, a cannabis researcher at King’s College London. “This can be a way in which the endocannabinoid system gets disrupted by heavy, long-term use of stronger cannabis, and is less able to function as it should.” People who develop problems from cannabis use may have less resilient endocannabinoid systems.

Why might weaker cannabis be less harmful?

Cannabis plants produce a second important substance called cannabidiol, or CBD. The compound appears to reduce the risk of psychotic disorders, by counteracting the effects of THC. But CBD and THC are made from the same precursor chemical, so maximising THC reduces CBD levels. This may have important implications. High-THC cannabis, such as skunk, appears to cause more mental health problems than low-THC cannabis, but the reason may be the lack of CBD to balance out the THC. A US study published this month found that as cannabis potency rose from 1995-2014, the ratio of THC to CBD rocketed from 14:1 to 80:1. Englund demonstrated the power of CBD in a striking study in 2014. He gave THC to volunteers and noticed they soon displayed acute psychotic symptoms. But when he gave them CBD too, they were largely protected from THC’s effects.

