You probably already know that taking aspirin can be helpful for preventing heart attacks-it's the foundation of Bayer Aspirin brand's entire advertising campaign. But you probably don't know that the now-infamous 1989 landmark study that cemented the drug's effectiveness in these situations included over 20,000 men-and zero women.

Why is this? For much of medical history, men (and male animals) have been the "guinea pigs" for testing-effects, dosages, and side effects have been measured on primarily or completely male subjects. In modern medicine, men have been the model; women are often an afterthought.

Unfortunately, the trend of overlooking the effects of medications in women continues today. In 2013, 20 years after the drug first became available, the Food and Drug Administration (FDA) cut the recommended dosage of Ambien for women in half (from 10 mg to 5 mg for the immediate release version). It turns out that women-5 percent of whom report using prescription sleep medications compared to just 3 percent of men-processed the drug more slowly than men, meaning they would feel drowsier during the day at the higher dose. This side effect comes with serious implications, including driving accidents.

Other research shows that women react to a wide variety of medications very differently from men. For instance, in one trial, male participants taking statins had significantly fewer heart attacks and strokes, but female patients did not show the same large effect. So it might, in fact, be harmful to prescribe statins-which often come with notoriously unpleasant side effects-to women with or without the risk of heart problems.

In some cases, women do better than men on SSRI antidepressants, and other research suggests that men have greater success with tricyclic drugs. Also, women who are addicted to cocaine show differences in brain activity compared to men, suggesting a mechanism by which women may become dependent on the drug more quickly. Therefore, leaving female models out of addiction studies, for example, potentially has serious implications for the drugs and standards of care that are later developed to serve addicts.

We also know that women show different symptoms in some serious illnesses. When women have heart attacks, for example, they may or may not feel the stereotype of chest pain. Instead, they are more likely than men to experience shortness of breath, cold sweat, and lightheadedness. Although sex isn't a factor in all aspects of health, when it is, it's often serious.

"We don't know yet whether [sex] is going to matter across the board in every illness, in every condition, but we need to know when it does matter," says Phyllis Greenberger, president and CEO of the Society for Women's Health Research. She was recently a part of a congressional briefing to discuss the role of sex differences in medical research, co-sponsored by her organization and The Endocrine Society.

Greenberger's organization was also integral to helping the 1993 NIH Revitalization Act pass, which required all National Institutes of Health (NIH) funded clinical trials to include women and minority participants. Currently, this group is one of many working to get the same consideration for the animals and cells used in preclinical research-not just humans.