A government study warned yesterday that marijuana poses great physical health risks than previously thought and that students are smoking it at an earlier age and in stronger doses than five years ago.

The report, released by the National Institute of Drug Abuse, said that since 1975 regular daily use of marijuana by high school seniors "has increased from 5.8 percent to 10.3 percent in 1979."

Moreover, three-fifths of the seniors had used it at least once. A high percentage, nearly a third, first smoked it before the 10th grade. The report said a 1977 survey showed that 43 million Americans of all ages had tried marijuana.

The report said that the "potency of marijuana in the United States has increased markedly over the years."

Before 1975 the report said, THC, the active ingredient, averaged below 1 percent in marijuana samples confiscated by the federal government, but in 1979 samples showed 5 percent concentrations to be common. The increased potency was attributed to less use of Mexican marijauna and increased use of stronger Columbian strains.

In the past, criticism of marijuana use has focused on momentary psychological and intellectual effects, and yesterday's report called attention to them, declaring that marijuana affects perception, motor coordination and memory, and can cause "impaired learning ability" when used by students during the day, as well as lessening coordination and skills needed for driving.

But the report emphasized that there is increasing evidence of physical damage, which most people thought to be of little danger a few years ago.

The report said it is likely that daily marijuana smoking leads to lung damage similar to that resulting from heavy cigarette smoking, and that "extended use of marijuana over a period of years may eventually be shown to cause cancer in humans."

It also said that one study comparing marijuana and cigarettes found that smoking less than one marijuana "joint" per day decreases the vital capacity (capacity to exhale) of the lungs "as much as smoking 16 cigarettes per day."

Another study "shows that marijuana smoke contains more cancer-causing agents than tobacco smoke. Another demonstrates that marijuana tar can produce tumors in animals," the report said.

All in all, it said, the "total body of clinical and experimental evidence" suggests that marijuana smoking leads to many of the same conditions as cigarette smoking, such as laryngitis, bronchitis, coughs and cellular change, and possibly cancer.

Effects on the reproductive cycle are also possible, the report said.

Studies have found marijuana correlated with decreases in levels of male sperm, decreases in the weight of the testes and ovaries, structural abnormality in the sperm, possible decreased fertility and abnormal menstrual cycles with failure to ovulate.

The report said there is some evidence that when a pregnant or nursing woman uses marijuana, THC and other substances are transmitted to the fetus or nursing child. In the fetus, it may concentrate in fatty tissues "including the brain."

The report said that while links to cancer and other conditions have not been established, research is showing marijuana as a "a complex drug which can negatively affect learning and motor coordination and may eventually lead to serious health problems."

It is not just "a simple herb with the power to enhance our lives," said Dr. William Pollin, director of the National Institute on Drug Abuse.

The report said marijuana has been shown to be effective in some cases in diminishing the nausea produced by cancer chemotherapy, and that, "while by no means invariably effective," this is "probably the single most promising application of marijuana.

Marijuana has also been shown to be effective to some cases in reducing vision-destroying pressure that occurs in open-angle glaucoma, and eye disease. But it said long term effectiveness and safety have not been established.