Marijuana is a controversial drug, widely used and widely illegal. Researchers in the Netherlands looked at one possible negative effect of the drug.

What the researchers wanted to know: Does marijuana use increase the risk of developing psychotic symptoms?

What they did: This was actually a prospective study, so the researchers looked at a few thousand German adolescents and young adults ages 14 to 24 in 1995, then followed up four years later to see what had changed, as part of a study on risk factors and mental disorders. That's a little bit stronger than the kind of study that just looks at a population to see if the people who use cannabis are the same people who have psychotic symptoms. It's not up to the level of a double-blind randomized controlled trial, but you might have trouble getting approval for an experiment where some kids smoke pot and some kids smoke a placebo to see which ones go psychotic.

What they found: People who had used cannabis at least five times before the 1995 interview had a higher risk of psychotic symptoms four years later. The more they'd used the drug, the higher the risk; those who used it almost daily were more than twice as likely to have psychotic symptoms later. People who were predisposed to psychosis, according to psychological tests in 1995, were not any more likely to be using cannabis four years later—important because one theory is that people with mental illnesses use marijuana and other drugs to self-medicate.

What the study means to you: This probably isn't a chance finding; there's also biological reasons to think that marijuana could work through endocannabinoid receptors to change your brain.

Find out more: Read about the federal penalties for trafficking marijuana and other drugs.

Read the article: Henquet, C., et al. "Prospective Cohort Study of Cannabis Use, Predisposition for Psychosis, and Psychotic Symptoms in Young People." British Medical Journal. Published online Dec. 1, 2004.

Abstract online: http://bmj.bmjjournals.com