Here is a summary of the major controversies about the health effects of caffeine: Birth Defects

Caffeine has long been under suspicion as a potential cause of birth defects, because it crosses the placenta and enters the bloodstream of the fetus. Since a fetus metabolizes nutrients slowly -- only about half the caffeine intake is eliminated within 20 hours -- it is exposed to caffeine longer and in higher concentrations than is an adult.

FDA studies in the late 1970s found that large doses of caffeine fed to pregnant rats caused birth defects -- mainly missing toes. Smaller doses -- equivalent to about two cups of coffee a day in humans -- caused some delayed skeletal development in the rats.

The evidence, while inconclusive, prompted FDA Commissioner Jere Goyan to warn in 1980 that "a prudent and protective mother-to-be will want to put caffeine on her list of unnecessary substances which she should avoid."

Goyan acknowledged that "we have no conclusive evidence at this time that caffeine has ever caused a birth defect in a human being" and that the study's implications for people are "not known" because humans may metabolize caffeine differently than rats do.

Since the FDA's official warning, two major human studies have been done. One, involving 2,030 malformed infants, found no correlation between a mother's consumption of caffeine and six common birth defects. The other, involving more than 12,000 women, showed no correlation between caffeine and low birth weight or infant malformations.

"It appears that the serious effects seen at high doses in animal studies cannot be extrapolated to the smaller doses that characterize usual caffeine consumption in the human diet," concluded Virginia Ernster, an epidemiologist at the University of California at San Francisco, in a recent review of the available data. Cancer

Caffeine has been implicated, by one study or another, in cancer of the bladder, kidneys and pancreas -- but the evidence is mixed.

A heavily publicized Harvard study reported in 1981 that coffee drinkers have an increased risk of pancreatic cancer. The study's chief investigator, Brian MacMahon, gave up coffee as a result but said he wouldn't presume to advise others.

But a review of the scientific literature by two physicians at Vanderbilt University Medical Center in 1983 criticized MacMahon's study for "serious flaws" and found no convincing evidence that caffeine causes cancer of the bladder, kidneys or pancreas.

"Available evidence does not suggest a recommendation against the moderate use of coffee," concluded the American Cancer Society in its 1984 nutritional guidelines. "There is no indication that caffeine . . . is a risk factor in human cancer." Fibrocystic Breast Problems

Two studies at Ohio State University in 1979 and 1981 were the first to suggest a possible link between methylxanthines (including caffeine) and fibrocystic breast disease -- a noncancerous condition of painful and lumpy breasts.

But other studies contradict that finding, and the Ohio State study has been severely criticized for many possible shortcomings, including its small number of subjects and lack of adequate controls.

A more recent study by researchers at the University of California at Berkeley looked at 138 pairs of female twins in which one sister had fibrocystic breasts and the other did not. The twin with the disease was likely to drink more coffee than her sister.

A review of the evidence by epidemiologist Ernster concluded that existing data "present an enigmatic picture" and that further study is necessary.

The AMA's Council on Scientific Affairs concluded in 1984 that "there is currently no scientific basis for associating methylxanthine consumption with fibrocystic disease of the breast."

Fibrocystic breast disease should not be confused with breast cancer, which has never been linked to caffeine. Caffeine and Children

Children rarely drink coffee, but their consumption of soft drinks and iced tea can expose them to significant levels of caffeine. Experts say caffeine's effects on children is a neglected area of research.

Average caffeine intake by children -- calculated in terms of body weight -- ranged from 36 percent to 58 percent of the intake by adults, the Federation of American Societies for Experimental Biology said in its 1978 report to the FDA. But a small number of children consumed as much as 10 times that average, or the equivalent of roughly 10 cups of coffee.

"The estimated levels of caffeine intake at these ages are near those levels that are known to cause central nervous system effects in adults," FASEB warned. Ulcers

For more than 40 years scientists have known that caffeine stimulates secretion of stomach acid and pepsin, which can aggravate existing ulcers.

But more recent studies have shown that both regular and decaffeinated coffee have the same gastric effect, suggesting that another ingredient besides caffeine also is involved. Doctors usually advise people with chronic ulcers or stomach irritation to avoid coffee during a flareup.

Coffee also has been shown to contribute to heartburn in some people by relaxing the muscle ring between the stomach and the esophagus, allowing stomach contents to back up and cause indigestion.

But given the variability of coffee's role in ulcers and heartburn, individuals may have to be the judge of their own stomach problems and alter their diets accordingly. Cardiovascular Disease

Caffeine can speed the heart rate and increase cardiac output, but studies are inconclusive about its role in heart disease.

A study in the early 1970s at Boston University Medical Center concluded that drinking one to five cups of coffee a day increased the risk of heart attack by 60 percent, and that more than six cups a day boosted the risk by 120 percent. And a Johns Hopkins study reported last November that a person who drinks five or more cups of coffee a day is nearly three times as likely to develop heart problems as a person who drinks no coffee.

But other studies have not substantiated those findings. The authoritative Framingham study, which has followed about 5,000 men and women since 1949, has found no correlation between coffee and coronary heart disease, angina (chest pain) or heart attack.

Because it can disturb or speed up the heart rhythms in some individuals, doctors advise people with heart disease or irregular heart rhythms (such as premature ventricular contractions) to limit or cut out dietary caffeine. The AMA's Council on Scientific Affairs concluded that key questions about caffeine's role in heart rhythm disturbances "remain unresolved."

Although caffeine can raise the blood pressure of non-habitual caffeine consumers, the rise is slight and temporary.

One group may suffer a particular risk from coffee-drinking, doctors warn. These are the people who feel the temptation -- during a coffee break -- to light up a cigarette.