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Do women ever respond differently than men to prescription drugs?

The fact that women take more medications than men increases their chance of suffering from drug interactions and side effects.

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The question

I'm interested in knowing whether women ever respond differently than men to prescription drugs. Do they?

The answer

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The short answer to your question is yes, in some cases. But it's taken the medical community a long time to reach that conclusion.

Traditionally, it was believed men and women react to medications in the same way. In fact, most research studies are carried out mainly on men. Even preliminary animal studies primarily use male rodents.

In some respects, the practice of excluding women from drug trials could be considered "well meaning," says Dr. Cara Tannenbaum, scientific director of the Institute of Gender and Health at the Canadian Institutes of Health Research in Montreal.

If a woman became pregnant during a study, her unborn child might be put at risk of birth defects. Many researchers were also striving for clear findings and they mistakenly believed that female hormonal fluctuations might complicate their results.

In recent years, however, there has been a growing awareness that women may not always respond in a similar fashion as men to certain drugs, Tannenbaum says. A key turning point occurred in January, 2013, when the U.S. Food and Drug Administration (FDA) issued new dosage recommendations for the commonly used sleeping medication zolpidem, sold under the brand names of Ambien and Sublinox. (Health Canada took similar action a year later.)

The regulators said the dose prescribed to women should be cut in half after studies revealed women metabolize, or break down, the drug more slowly than men.

Women could wake up with high levels of the drug still in their bloodstream, which might impair their ability to drive or do other tasks requiring an alert mind.

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This was the first time regulators recommended sex-based dosages for an oral medication.

Since then, the FDA and Health Canada have called on researchers to include more women in clinical trials.

There are certainly good biological reasons to think women and men may handle some medications differently, says Dr. David Juurlink, a drug-safety expert and head of Clinical Pharmacology and Toxicology at Sunnybrook Health Sciences Centre in Toronto.

He notes that drugs have to be absorbed, metabolized and eventually eliminated from the body – processes that involve several different organs. There can be subtle differences in how these organs function depending on a person's sex.

For instance, the activity levels of enzymes that metabolize drugs in the liver can sometimes differ in men and women, Juurlink says. Furthermore, women tend to have a higher percentage of body fat than men – and certain drugs can accumulate in fat tissue. That means some drugs are more likely to build up in the bodies of women than men.

Aside from biology, the medical profession also treats the sexes differently. "Women are more often prescribed medications such as sedatives, sleeping pills, opioids and antidepressants," Tannenbaum says. The fact that women take more medications than men increases their chance of suffering from drug interactions and side effects.

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A study conducted by the U.S. Government Accounting Office suggests women do indeed face elevated risks from some medications.

Between 1997 and 2000, the FDA pulled 10 drugs from the market for safety reasons. A data review by the accounting office found eight of "the withdrawn drugs posed greater health risks for women than for men."

These problems might have been avoided if more females had been included in the research – starting with the basic science involving laboratory animals.

In a groundbreaking study published last year, a team of researchers revealed that pain is processed in male and female mice using different cells.

Earlier work, done exclusively on male rodents, concluded that the transmission of pain signals in the spinal cord depends on an immune cell called microglia. But when the researchers did the same experiments using female mice, they discovered that another type of immune cell – called a T-cell – appears to be responsible for sending the pain signals.

"My expectation was that there was going to be no difference between the male and female mice," says one of the authors of the study, Dr. Michael Salter, chief of research at the Hospital for Sick Children in Toronto. "I was certainly surprised by the result."

Their findings could have far-reaching implications, suggesting that pain medications may need to be focused on different mechanisms in men and women, Salter says.

So, should women be concerned every time they pop a pill? Not necessarily.

Doctors still expect women and men will have the same response to most drugs. But for a select group of medications, the differences could be significant. The best way to identify those drugs is by routinely including women in research.

As Salter puts it: "Unless you actually ask the question – 'Is there a difference?' – you are never going to know the answer."

Paul Taylor is a patient-navigation adviser at Sunnybrook Health Sciences Centre. He is a former Health Editor of The Globe and Mail. You can find him on Twitter @epaultaylor and online at Sunnybrook's Your Health Matters.

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