Skin tags -- those small, harmless flaps of skin that often grow on the neck or eyelids in people over 40 -- may be a sign of intestinal polyps, a new study suggests.

Researchers at Mount Sinai School of Medicine in New York suggest that people with skin tags should be tested for blood in the stool, which can reveal polyps, because polyps can become cancerous.

Writing in the Journal of the American Academy of Dermatology, Dr. Mark Lebwohl reports that among 54 people studied, 86 percent of those with polyps also had skin tags.

Skin tags, which are nonmalignant, also are commonly found on the shoulder and in the groin area. They often grow on the bodies of diabetics and pregnant and postmenopausal women. Polyps, on the other hand, while usually benign, can become malignant.

Lebwohl pointed out that in his study, most of the patients had some form of stomach or digestive trouble before they were examined, so further study is necessary before a connection between skin tags and polyps can be made for the general population. Doctors Who Treat AIDS Feel Depressed, Anxious

Doctors who treat AIDS patients complain of more depression, anxiety and fear of death than they had before the onset of the epidemic, a survey of 82 San Francisco physicians concludes.

More than one third reported "more or much more" depression than before, 56 percent repored increased stress, and almost half reported "more fear of death . . . since they began working with patients with AIDS," Dr. Leon McKusick of the University of California at San Francisco writes in the Western Journal of Medicine.

Increased anxiety was especially common among homosexual doctors, partly a result of "the gay physicians' self-perception as being at risk for AIDS," McKusick and his colleagues said.

Many physicians feel stress because they see an increasing number of deteriorating patients "without having adequate recommendations for treatment." Of the approximately 21,000 Americans diagnosed with acquired immune deficiency syndrome since 1977, more than half are dead.

Despite the pressures, almost half of the doctors reported more career satisfaction since beginning work with AIDS patients, although some found it necessary to take breaks from their practice.

"I get away on vacation more often," one doctor said, "far enough away to realize how depressed everyone is in San Francisco and how the rest of the world is still going along as usual." New Drug Combination Tried on Brain Tumors

A new combination of drugs shows promise in reducing brain tumors, traditionally difficult to treat with medications because the body tries to protect the brain from foreign substances.

The new approach, devised at Yale University, combines a drug that fights cancer with another drug, often used on psychotic patients, that passes through the body's so-called blood-brain barrier.

The anti-psychotic drug, phenothiazine, seems to limit the growth of cancer cells, thereby enhancing the effectiveness of the small amount of cancer-killing drug that makes it into the brain.

Dr. William N. Hait and colleagues at Yale tried the combination of drugs on 18 patients who had stopped responding to traditional chemotherapy, primarily to check for side effects. The patients had glioblastoma multiforme, a usually fatal form of cancer, which is treated by surgery, then radiation and chemotherapy to kill the remaining cancer cells intertwined with brain tissue.

The preliminary results of that trial are "promising," Hait said. But additional research is needed before the technique will be ready for general use. Some Diabetes Reversed Among Native Americans

When the Zuni Indians of western New Mexico became urbanized in the 1940s and 1950s and their lives grew sedentary, diabetes became rampant. Today, an estimated 40 percent of the adults are diabetic.

A two-year study of those Native Americans has found that such adult-onset diabetes, when the result of life style, can often be tempered or reversed with exercise and changes in diet.

Similar findings have been reported among other American Indian populations, and among aboriginal Australian people.

Among the Zuni, the U.S. Public Health Service reports in its official journal, diabetes developed as life style "suddenly became more sedentary as agriculture, stock-raising, hunting, hand labor, foot transportation, and foot-racing gave way to jewelry making and marketing, mechanization, motor vehicle transportation and television."

Diet also changed from high-fiber to high-fat. "In a traditionally feast-or-fast culture, feast conditions, in effect, now exist year around," Dr. Robert Wilson of the Zuni hospital writes with several colleagues.

In 1983, Wilson's team started the Zuni Diabetes Project, an exercise and diet program aimed at fighting or preventing diabetes. Most of the 35 diabetics in the program were able to reduce or totally eliminate use of insulin.

Adult-onset diabetes, they conclude, "can often be controlled or prevented by losing weight through regular aerobic exercises and decreased calorie intake."

But, they note, "It is difficult to motivate people to modify their activity levels and eating habits." On the Pulse

Small, temperature-sensitive patches that are placed on the forehead to detect fevers are just as accurate as the electronic thermometers used in most emergency rooms, an Arizona study concludes. But hospitals probably won't be switching en masse to the new technology. Each patch costs about 60 cents and can be used only once, while the disposable sanitary sheathes for electronic thermometers costs only a penny or two, the American Journal of Nursing reports . . . One of the toughest things about being a hospital intern, it is commonly believed, is that there is little time to sleep. Not so, says a study in the Southern Medical Journal. "Our findings indicate that there is sufficient time during the internship for physiologically adequate sleep," write Drs. Charles Ford and Dennis Wentz, "but insufficient time for practically anything else" . . .