From an interesting Slate article by Melinda Wenner Moyer:
According to a 2006 study in the Journal of Women’s Health, women made up less than one-quarter of all patients enrolled in 46 examined clinical trials completed in 2004. And although more women than men die from heart disease each year, a 2008 study published in the Journal of the American College of Cardiology reported that women comprised only 10 percent to 47 percent of each subject pool in 19 heart-related trials.
The pro-male bias even extends to animals:
According to a study in-press in Neuroscience and Biobehavioral Reviews, out of nearly 2,000 animal studies published in 2009, there was a bias toward the use of male animals in eight of 10 disciplines. Neuroscientists used 5.5 males for every one female, pharmacologists used five, and physiologists used 3.7. And despite the fact that women are twice as likely to suffer from major depression, fewer than 45 percent of animal studies on these disorders used females.
And the bias matters:
Women taking antidepressants and antipsychotics tend to have higher drug concentrations in their blood than men do; they also require half as much influenza vaccine for the same level of protection, though they are always given the same amount. In an analysis of 11 new drug applications submitted to the Food and Drug Administration between 1995 and 2000, drug concentrations in blood and tissues from men and women varied by as much as 40 percent. However, the applications included no sex-based dosing recommendations. Finally, women are more likely than men to experience adverse drug reactions: Eight out of 10 prescription drugs pulled from the U.S. market from 1997 to 2001 were banned because of the side effects they caused in women alone. [Note: Four of these were because more women than men took the drugs.]
Wenner Moyer says the overemphasis on males partly stems from laziness — men are easier to study because their sex hormones don’t fluctuate in the same way — and is partly a legacy of a 1977 FDA ban on women who could become pregnant participating in early-stage clinical trials, which in practice ended up being applied to many other women. The law was superseded in 1993, although the bias against pregnant women remained.