Every year, about 1,300 Americans, half of them children under age 14, die from injuries sustained in bicycle accidents. The leading cause of death in such accidents is head trauma.

In most cases, experts say, those deaths could be prevented if bicyclists wore safety helmets. But they don't. Less than 2 percent of elementary, junior high and high school students wear a helmet when they ride a bicycle.

Although physicians are aware of both the risk of bicycle riding and the tendency of children not to wear helmets, most never raise the issue in counseling patients, according to a new study by two doctors at the University of Arizona College of Medicine.

In a survey of 90 pediatricians and family physicians in the Tuscon area, Drs. Barry D. Weiss and Burris Duncan found that most of the doctors understood that bicycle accidents are a high-ranking cause of death among children. Ninety-two percent also correctly estimated that less than 5 percent of children use bicycle helmets.

But only 1 percent of the pediatricians and family physicians discuss bicycle safety with patients "routinely," the survey found. More than 29 percent "never" do, and another 40 percent "almost never."

Family physicians were slightly more likely than pediatricians to report never discussing bicycle safety.

"Despite awareness of the problem, it appears that physicians do not regularly include bicycle safety in the educational information they provide to patients and parents," wrote Weiss and Duncan in next month's American Journal of Public Health.

Family physicians and pediatricians, concluded Weiss and Duncan, should broach the subject of bicycle helmet use not only with children but with their parents. Therapy Plus Methadone Helps Curb Heroin Addiction

Heroin addicts are more likely to kick their drug habit if they receive outpatient psychiatric therapy in addition to methadone, reports a study in next month's Archives of Psychiatry.

The reason: They are more able to cope with depression and life crises, such as losses caused by divorce, separation or death. Such crises often trigger relapses in drug abusers.

"Opiate addicts maintain very high levels of life crises, and those addicts who continue to abuse drugs have significantly more life crises than those who remain abstinent," said Dr. Thomas R. Kosten, who headed the study at Yale University School of Medicine.

The importance of the Yale study lies in its demonstration that not all drug addicts suffer from the same problems, said Dr. Roger E. Meyer, of the University of Connecticut Health Center in Farmington, Conn. Treatment must take into account emotional problems and differences in psychological outlook as well as drug abuse, he said in an article in the current Journal of the American Medical Association.

The 268 addicts in the Yale study were evaluated not only for drug use, but also for psychiatric illness and psychosocial behavior, and were treated accordingly.

"It is paradoxical," Meyer said, "that federal support for outpatient services for drug-dependent patients has been curtailed, while inpatient services have become more available for the treatment of these chronic, relapsing disorders." High-Sugar Diets Might Prompt Appendicitis

An unexplained outbreak of appendicitis in a small Texas town reminds doctors that they don't know the cause of the disease, although the treatment is rather straightforward.

The outbreak of 13 cases, reported recently in the American Journal of Surgery, took place between February and April of 1984. The town had only two cases during those months the previous year.

An inflamed appendix must be surgically removed. It is most common in people between 10 and 29, and strikes males 50 percent more often than females. Viruses, bacteria and parasites have been proposed as possible causes, but nothing has been proved, although it is not believed to be contagious.

Diet also is believed to be a factor. In the Texas cases, doctors theorize that high-sugar diets may have made the victims vulner- able (10 were school-age boys), and a bacterium or virus in farm eggs may have triggered the outbreak. Human Obesity Is Under Strong Genetic Control

A study of more than 4,000 pairs of male twins has confirmed earlier findings that heredity is an important factor in obesity.

The study, reported this month in the Journal of the American Medical Association, compared identical twins, which originate from a single egg at fertilization, with fraternal or nonidentical twins, which are conceived from two fertilized eggs. It is believed to be the largest study of its kind.

Analyses of the results, the researchers concluded, "all suggest that human fatness is under strong genetic control."

The study, by researchers at the University of Pennsylvania School of Medicine, looked at 1,974 sets of identical twins and 2,097 sets of nonidentical twins. The sample was drawn from the National Academy of Science-National Research Council's registry of nearly 16,000 twin pairs who were born between 1917 and 1927 and served in the armed forces in World War II or in Korea.

Identical sets of twins were much more likely than nonidentical twins to show the same degree of obesity, both at the time of their induction and 25 years later, the study found. (Since identical twins are genetically the same, any difference in their physical makeup would be attributed to environmen- tal factors.) Height, weight and body mass index (a measure of fatness) all were found to be "under substantial genetic control." Most Ob-Gyns Have Considered Leaving Obstetrics

Nearly two thirds of the gynecologists and obstetricians surveyed by McCall's magazine this month say they have either left obstetrics or considered leaving it because of the malpractice crisis.

Sixty-two percent of the gynecologists and obstetricians surveyed said they had been sued for malpractice. One in four are performing more Cesarean sections, 19 percent have left obstetrics and 46 percent have thought about leaving the field.

The results are based on answers to a questionnaire sent to a representative national sampling of gynecologists and obstetricians. A total of 616 doctors, including 542 men and 74 women, responded.

Other findings in the McCall's survey: Almost all the doctors defended a woman's right to have an abortion. More than four out of five provide abortion counseling and referral services to patients, and only 6 percent say abortions should never be performed. The doctors split on the question of whether too many hysterectomies are being performed: 48 percent said yes, and 46 percent said no. The method of birth control most widely used by gynecologists and their partners is "the pill" -- 55 percent use it and 91 percent recommend it to patients. Sterilization was second -- 18 percent had been sterilized. Only 5 percent use the IUD, and 56 percent discourage its use by patients. Premenstrual syndrome (PMS) is not "all in your head," the doctors said. Ninety-five percent said they believe that the anxiety, mood swings and discomfort of PMS are real. Three out of four doctors said pregnant women who have merely had contact with a person with AIDS should be screened for the AIDS virus. Four out of five said women who have had sexual relations with bisexual men should be tested.On the Pulse

Beware of cookie and cracker labels boasting that the products are "made with 100 percent vegetable shortening," warns the Center for Science in the Public Interest. As healthful as that sounds, these products often contain palm or coconut oils -- which are more saturated than lard and equally harmful to the heart . . . Credit card medicine is on the rise, Medical Economics reports. In the past two years, the number of hospitals, doctors and other health providers who accept the American Express card for payment has jumped from about 2,000 to about 16,000 . . . Amish children in Pennsylvania have half as many dental cavities as the average American child, researchers at the University of Michigan report. The finding is puzzling, because Amish children tend to consume large amounts of sugar and drink unfluoridated water . . . Fifty-five percent of all hospital patients and 58 percent of those over 65 were malnourished at the time of admission, according to a study of 771 hospital cases by the American Dietetic Association and Ross Laboratories. Compared with well-nourished patients, the malnourished ones had three times as many complications and stayed in the hospital two to five days longer . . .