Pot on the spot: marijuana's risks become blurrier.

The federal government's war on drugs gets plenty of ammunition from scientific studies that have correlated the use of such substances to various psychological problems. Conspicuously absent, however, are data showing that marijuana, one of the most widely used illicit drugs, causes mental or behavioral problems in teenagers and young adults, a new report concludes.

The causal chain of events could just as easily run in the opposite direction, suggest psychologist John Macleod of the University of Birmingham in England and his colleagues in the May 15 Lancet. Available evidence is consistent with the possibility that various psychological and social difficulties foster marijuana use, which may then contribute to a worsening of those problems, Macleod's group contends.

After reviewing 48 relevant multiyear studies published between 1975 and 2003, the team focused on 16 investigations that had regularly assessed large samples of children or teenagers for at least 10 years.

"We've found no strong evidence that use of [marijuana] in itself has important consequences for psychological or social health, but we cannot exclude the possibility that such a relation exists," Macleod says.

He and his colleagues are particularly skeptical of recent reports from Sweden and New Zealand that around 1 in 10 teenagers who had smoked marijuana experienced schizophrenia symptoms by young adulthood. It's doubtful that marijuana plays a direct role in schizophrenia, Macleod's group argues, because the mental disorder's worldwide incidence has remained stable while the proportion of teens reporting marijuana use has fluctuated.

Psychiatrist Herbert D. Kleber of Columbia University says that this argument underplays the increased risk of schizophrenia reported in the Swedish and New Zealand studies. There's now so much evidence of an association between teens' marijuana use and later psychosocial problems that it's hard to dismiss the likelihood of a causal effect, Kleber argues. "Macleod's team sees the smoke but won't acknowledge that there's a fire," he says.

The controversy continues to smolder. The new review of research results "confirms what's been known for decades about marijuana's lack of extreme harmfulness," remarks medical sociologist Marsha Rosenbaum, director of the Drug Policy Alliance's San Francisco office. Her organization works to decriminalize marijuana but doesn't condone its use by teenagers.

On the other hand, David Murray, special assistant to the director of the Office of National Drug Control Policy in Washington, D.C., argues that reports of teens often developing psychological or behavioral difficulties after beginning to smoke marijuana are reason enough to regard early use of the drug as a public health concern, especially given the increased potency of marijuana sold in the United States in recent years.