Cannabis users suffer mental health problems twice as often as nonusers, a study published Monday by Statistics Netherlands said.
"The percentage of people with less than good mental health is two times higher among cannabis users than nonusers," the Central Bureau of Statistics (CBS) said in a statement.
The study, which was carried out between 2007 to 2009 and involved some 18,500 people, found that "20 percent of male cannabis users complained of mental problems, compared to 10 percent among nonusers."
The proportion was similar for women, with 28 percent of users complaining of mental problems, compared to 14 percent among nonusers, CBS said.
Smokers complained of mental problems such as anxiety, melancholy, sadness, and impatience, but CBS said the physical health of users and nonusers barely differed.
The study found that four percent of the 15- to 65-year-olds who took part had smoked cannabis at least once in the last 30 days.
Though technically illegal, the Netherlands decriminalized the consumption and possession of under 5 grams (0.18 ounces) of cannabis in 1976 under an official "tolerance" policy.