The known and suspected health hazards of marijuana justify "serious national concern" about the most widely used illicit drug in the United States, a prestigious panel of medical experts reported yesterday.

Although evidence of permanent, long-term adverse effects is still lacking, a National Academy of Sciences committee concluded that there is a "demonstrated high potential of risk to human health."

It cited short-term effects on behavior and learning that may have serious effects on schoolchildren, but found no "conclusive evidence" that prolonged use causes permanant changes in the nervous system or brain.

The 22-member group did suggest that long-term marijuana smoking could lead to lung damage and cancer and raised questions about "worrisome"--but not definitive--findings on possible adverse reproductive effects.

The Academy's Institute of Medicine panel warned that the already small amount of federal financing--about $4 million annually--for marijuana studies has been diminished by inflation in recent years and called for a "greatly intensified and more comprehensive program of research."

"Our major conclusion is that what little we know for certain about marijuana's effects on human health--and all that we have reason to suspect--justifies serious national concern," said Dr. Arnold S. Relman, committee chairman and editor of the New England Journal of Medicine.

"We cannot rule out any risk to health in any area," he added at a crowded press briefing. "Any prudent person looking at the facts would have to be worried about the long-term effects."

Of particular concern, he noted, is the daily use of the drug by one in 14 high school seniors. The number has declined recently, in part because of increased recognition by students of possible health risks. Estimates suggest that about one quarter of the entire American population has tried marijuana.

The 15-month, $450,000 study is perhaps the most extensive independent review yet of the available scientific evidence on marijuana's health effects. The findings of the government-funded effort include:

* Marijuana use impairs motor coordination, posing a "substantial risk" for anyone attempting to drive. These effects could last at least four to eight hours after the actual "high."

* The drug interferes with short-term memory and may produce effects ranging from euphoria to delirium. The group found it difficult to determine whether marijuana use was a cause or effect of the so-called "amotivational syndrome," which produces apathy and poor school or work performance.

* Marijuana causes changes in the heart and circulation that could threaten patients with these problems, but not normal individuals.

* There is a "strong possibility" that prolonged, heavy smoking of marijuana will lead to cancers of the respiratory tract, similiar to those caused by tobacco.

* A chemical in marijuana is known to reduce the number and movement of sperm in men, but the effect on fertility is unknown. The panel expressed concern about "young girls using the drug" because of animal research showing effects on ovulation and reproductive hormones.

* Evidence conflicted as to whether marijuana affects the immune system.

* Marijuana and its byproducts can remain in the body for long periods of time, even months, with unknown, but possibly "subtle" effects.

"Marijuana has a broad range of psychological and biological effects, some of which, at least under certain conditions, are harmful to human health. Unfortunately, the available information does not tell us how serious this risk may be," the panel report said.

On the positive side, the new review found that marijuana could be potentially beneficial in the treatment of glaucoma, the nausea brought on by cancer chemotherapy, asthma and perhaps certain types of epileptic seizures, spastic disorders and other nervous system diseases.

Relman noted that the academy review, which focused on published scientific reports, failed to find marijuana either as safe or as dangerous as some have claimed: "Our committee found the present truth of the matter to lie somewhere in between the two extremes, so we give no comfort to those with strong positions on either side of the argument."

The report came under criticism from the National Federation of Parents for Drug-Free Youth. "We are concerned this prestigious organization has not been able to come down with a stronger warning, which is what our children need," said spokesman Joyce Nalepka.

But George Farnham, of the National Organization for the Reform of Marijuana Laws, called it an "unbiased, unslanted analysis" which "debunked some of the 'Reefer Madness' hysteria stories of the last few years."