For example, Seldane, an antihistamine, and the gastrointestinal drug Propulsid both triggered a potentially fatal heart arrhythmia more often in women than in men. Many drugs still on the market cause this arrhythmia more often in women, including antibiotics, antipsychotics, anti-malarial drugs and cholesterol-lowering drugs, Dr. Clayton said. Women also tend to use more medications than men.

The sex differences cut both ways. Some drugs, like the high blood pressure drug Verapamil and the antibiotic erythromycin, appear to be more effective in women. On the other hand, women tend to wake up from anesthesia faster than men and are more likely to experience side effects from anesthetic drugs, according to the Society for Women’s Health Research.

Women also react differently to alcohol, tobacco and cocaine, studies have found.

It’s not just because women tend to be smaller than men. Women metabolize drugs differently because they have a higher percentage of body fat and experience hormonal fluctuations and the monthly menstrual cycle. “Some drugs are more water-based and like to hang out in the blood, and some like to hang out in the fat tissue,” said Wesley Lindsey, assistant professor of pharmacy practice at Auburn University, who is a co-author of a paper on sex-based differences in drug activity.

“If the drug is lipophilic” — attracted to fat cells — “it will move into those tissues and hang around for longer,” Dr. Lindsey added. “The body won’t clear it as quickly, and you’ll see effects longer.”

There are also sex differences in liver metabolism, kidney function and certain gastric enzymes. Oral contraceptives, menopause and post-menopausal hormone treatment further complicate the picture. Some studies suggest, for example, that when estrogen levels are low, women may need higher doses of drugs called angiotensin receptor blockers to lower blood pressure, because they have higher levels of proteins that cause the blood vessels to constrict, said Kathryn Sandberg, director of the Center for the Study of Sex Differences in Health, Aging and Disease at Georgetown.

Many researchers say data on these sex differences must be gathered at the very beginning of a drug’s development — even before trials on human subjects begin.

“The path to a new drug starts with the basic science — you study an animal model of the disease, and that’s where you discover a drug target,” Dr. Sandberg said. “But 90 percent of researchers are still studying male animal models of the disease.”

There have been improvements. In an interview, Dr. Robert Temple, with the Center for Drug Evaluation and Research at the F.D.A., said the agency’s new guidelines in 1993 called for studies of sex differences at the earliest stages of drug development, as well as for analysis of clinical trial data by sex.

He said early research on an irritable bowel syndrome drug, alosetron (Lotronex), suggested it would not be effective in men. As a result, only women were included in clinical trials, and it was approved only for women. (Its use is restricted now because of serious side effects.)

But some scientists say drug metabolism studies with only 10 or 15 subjects are too small to pick up sex differences. Even though more women participate in clinical trials than in the past, they are still underrepresented in trials for heart and kidney disease, according to one recent analysis, and even in cancer trials.

“The big problem is we’re not quite sure how much difference this makes,” Dr. Lindsey said. “We just don’t have a good handle on it.”