The debate over the longterm effects of marijuana is far from over, but more and more authorities are weighting in on the "it's dangerous" side. The latest is the American Medical Association, which warns that frequent pot use over prolonged periods can lead to serious problems in the brain, circulatory system, heart, lungs and nervous system.

The AMA said recent studies indicate concentrations of THC, the active ingredent in marijuana, are steadily increasing in pot users -- from 1 percent in 1970 to 5 percent in 1979 -- and pose a threat to vital organs.

"Structural changes occur in the brain with marijuana use, as well as changes in the patterns of brain waives," the Ama says in its new drug abuse handbook for physicians. "Acute marijuana intoxication impairs learning, memory, thinking, comprehension and general intellectual performance. Even at moderate levels of social use, driving skills are impaired."

The AMA also says that marijuana contains higher concentrations of cancer-causing hydrocarbons than tobacco and that daily use can lead to bronchitis and emphysema. In women, heavy marijuana use can lead to disruptions of the menstrual cycle, temporary infertility and miscarriages, the organization adds, and in men it can lead to sperm abnormalities and damage to the reproductive organs.

In addition, heavy pot smokers may suffer from psychological dysfunction, and personality and emotional problems.

"The psychological damage may be permanent," the AMA warns. "Large doses of THC can induce hallucinations, delusions and paranoid feelings. Thinking becomes confused and disoriented. The initial euphoria may give way to anxiety reaching panic proportions."

The AMA notes marijuana is the third most frequently used drug in the United States, behind only alcohol and cigarettes. Two-thirds of young adults say they use the substance and many are combining marijuana and alcohol, posing a hazard of more widespread and severs reactions to the combined effects of the drug, it says.