Nearly four percent of adults around the world use cannabis, even though the drug raises many major health concerns, according to a paper published in The Lancet on Friday.
It cited figures from the UN Office on Drugs and Crime, which estimated that in 2006 there were 166 million users of cannabis aged from 15-64, or 3.9 percent of the world's population in this age category.
The drug is most used among young people in rich countries, led by the United States, Australia and New Zealand, followed by Europe, but appears to be becoming popular on a global scale, with use rising in low- and middle-income countries, it said.
The study, written by Australian professors Wayne Hall and Louisa Degenhardt, is an overview of published research into cannabis use and impacts.
Hall and Degenhardt say that, as a problem for public health, cannabis use "is probably modest" compared with the burden from alcohol, tobacco and other illegal drugs.
Even so, cannabis has a long list of suspected adverse health effects, they warn.
These include the risk of dependence, car accidents, impaired breathing, damaged cardiovascular health, psychotic episodes and educational failure among teens who smoke the drug regularly.
Around nine percent of people who ever use cannabis become dependent on it, says the paper. By comparison, the risk of addiction for nicotine is 32 percent, 23 percent for heroin, 17 percent for cocaine and 15 percent for alcohol.
"Acute adverse effects of cannabis use include anxiety and panic in naive [first-time] users, and a probable increased risk of accidents if users drive while intoxicated," it says.
"Use during pregnancy could reduce birthweight, but does not seem to cause birth defects. Whether cannabis contributes to behavioural disorders in the offspring of women who smoked cannabis is uncertain."
Those who claim that cannabis is safer than smoking are also probably misguided.
Cannabis smoke contains many of the same cancer-causing chemicals as tobacco smoke, and some of them are in higher concentrations. Wheezing, coughing and bronchitis are commonly reported amongst cannabis smokers.
Evidence to support suspicions that cannabis causes cancer are inconclusive, though, because most frequent and long-term cannabis users also smoke tobacco.
Another area of concern is about so-called skunk -- extremely potent cannabis that derives from plants selected to have higher levels of tetrahydrocannabinol (THC), the drug's active ingredient.
Some evidence has emerged that levels of THC found in seized cannabis have risen in the past two decades, says the study.
"A high THC content can increase anxiety, depression and psychotic symptoms in naive users, while increasing the risk of dependence and psychotic symptoms if regular users do not titrate [measure out] their dose."
(c) 2009 AFP
Explore further: Study investigates cannabis use among university students