Taking more than one medication at a time is always a risky business, since it increases the likelihood of an adverse drug reaction (ADR) -- a harmful, unintended, or unwanted effect of a medication. In the United States, ADRs are a leading cause of illness and account for numerous deaths each year. (How many deaths do ADRs cause?) 

According to the results of a number of recent studies, it's even more of a danger for women, in part because they are more likely to use multiple medications and dietary supplements. Sex-based differences in metabolism, anatomy, and hormone levels also seem to play a role in why women experience more unwanted effects of drugs.

"Recent findings suggest women are significantly more likely to suffer from adverse drug interactions because of their biology and likelihood of taking more than one medication," said Phyllis Greenberger, president and CEO of the Society for Women's Health Research. "Women should think twice before combining even commonplace products like ibuprofen, St. John's Wort and oral contraceptives with other drugs."

A recent review of 48 studies in the United Kingdom revealed that adverse drug reactions to newly marketed drugs are 60% more common in women than in men. This sex difference was observed across all age groups older than 19 years old. Other studies have found less pronounced, yet significantly higher risks of ADRs among women.

Women need not look further than their own medicine cabinet for potentially problematic combinations, experts warn. For instance, oral contraceptives can fail resulting in pregnancy when taken in combination with the antibiotics rifampin, tetracycline or penicillin. Some antacids can inactivate fluoroquinolones, drugs often prescribed for urinary tract infections, allowing infections to progress unchecked. In addition, combining selective serotonin reuptake inhibitors (SSRIs) with other types of antidepressants or pain medications can have serious consequences.

Their greater use of multiple drugs is not the only reason women are at an elevated risk of adverse drug reactions. Research points to a number of other possible mechanisms for this sex-based difference. Variations between men and women in liver and kidney functioning, which affects drug metabolism, as well as anatomy seem to influence the incidence of adverse drug reactions.

On average, women have a lower body weight, smaller organ size, reduced blood flow and a higher proportion of fat compared with men. Overall differences in hormonal activity between the sexes affect the way drugs are processed, absorbed and cleared by the body as well. Because of these sex-differences, the optimally safe and effective dose of a drug varies between men and women. Research suggests that a significant percentage of adverse drug events among women may be due to unnecessarily high doses of frequently prescribed drugs.

What's more, women should refer to a drug reference book, if possible, according to Maryann Napoli, associate director of the Center for Medical Consumers in New York City. Almost all local libraries have a drug reference book in their collection and there are also some reliable sources of drug information on the Internet. Napoli suggests women get in the habit of reading up on every prescription or over-the-counter drug before they start using it.

SOURCE: "Mixing Medications Can Spell Trouble for Women," Society for Women's Health Research August 23, 2002.