People who smoke marijuana daily or weekly double their risk of developing a psychotic illness over their lifetime, according to a study published Thursday in the medical journal The Lancet.

Among all cannabis users, including sporadic experimenters and habitual users, the lifetime risk of psychotic illness increased by 40 percent, the report said.

“It’s not as if you smoke a joint and you’re going to go crazy,” said Richard Rawson, who directs the Integrated Substance Abuse Program at the University of California, Los Angeles, and was not involved in the study.

But he cautioned: “It’s definitely not a good idea to use heavy amounts of marijuana.”

The researchers found that the risk for psychotic illnesses did appear to increase with dose, suggesting that stopping marijuana use would decrease risk, said co-author Dr. Stanley Zammit, a psychiatrist at Cardiff University and the University of Bristol in Britain.

Psychotic illnesses include schizophrenia and disorders with symptoms such as hallucinations or delusions. Zammit estimated that they develop in about 3 percent of the population.

Marijuana is the most commonly used illicit drug in the U.S., according to the federal government. An annual study funded by the National Institute on Drug Abuse found that about 42 percent of America’s high school seniors in 2006 had tried marijuana at least once in their lives.

Marijuana can cause psychiatric problems because it throws off the balance of neurotransmitters in the brain, Zammit said.