There is increasing evidence that research on men is often not applicable to women. Clinical trials traditionally use males to avoid liability for affecting possible pregnancies. Women are also excluded for fear hormonal cycles interfere with results, despite this being an important factor in determining whether drugs are safe. As we learn of more consequences of biological sex differences, such as different drug reactions or susceptibility to various diseases, it becomes problematic – even unscientific – to rely on data derived mainly or exclusively from men. It follows that relying on results of studies derived from animal data is even riskier. From 2000-2003, only 3% of studies funded by the National Institutes of Health, the largest scientific granting body in the U.S., included investigation into sex differences. There is a need for further exploration, as research overly relies on male data that may or may not be applicable to females, or data derived from animal studies that are inapplicable to both.

There is huge variety in the types of sex differences discovered to date. For example, women are 3.5 times less likely than men to suffer brain tumors. Women often have lower blood pressure, yet hypertension treatments are less effective than for men. A lack of attention to sex differences becomes dangerous when predicting drug reactions. Because men and women do not metabolize drugs in the same manner, factors like dosage requirements and side effects can differ greatly. Such differing drug reactions are not trivial, and can be life threatening. For the cardiac drug d-Sotalol, the risk of death in women was 2.5 times greater than in men. Sex differences in other species have received even less attention, further amplifying the case against applying data from one species to another.

Many drugs that appear safe and effective in animals fail in humans, and often cause significant harm, even death. A recent analysis reported 94% of drugs that enter clinical trials following animal testing fail to be approved. Of the remaining drugs that are approved, half are withdrawn or relabeled due to severe or lethal adverse effects not detected during animal testing. The results of toxicity studies in animals show considerable disparity between species, and are not reliably predictive for humans. All of the more than 85 AIDS vaccines successful in non-human primates failed in human trials. The application of animal studies and predominantly male-based data onto human health ignores the unique female genetic makeup affecting women's health issues and the profound differences between species.

It is clear men and women are significantly biologically distinct, often suffer differently from the same diseases, and process drugs differently. It is imperative that knowledge of sex variability be factored into research, drug development, and medical practice. Yet science and medicine are not adequately responding. Remedies to this bias include changes of attitude and priorities in scientific and medical arenas, and, importantly, more women in senior levels of science. Because it is widely acknowledged we are entering an age of personalized medicine, sex variability needs prominent consideration, and it is folly to continue our dependency on other species as models for humans. Research on non-human animal models is not working and for progress in combatting human disease, it must be abandoned in favor of human-based methods. These methods must incorporate sex differences. Leaving animal models behind and bringing the study of women to a focus equivalent to that of men will better lead to the medical breakthroughs we all await.

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