As recently as the 1990s, the fear of women’s hormone cycles skewing results of biomedical studies led researchers to rely on the use of male animals and humans during experimental drug testing.

As more drugs hit the market, instances of women having complications from tested and approved drugs prompted the National Institutes of Health and Congress to require that women be included in research involving human subjects, officially named the Revitalization Act.

Now, according to the NIH, women account for 57 percent of enrollees across all of its funded clinical research studies.

Janine Clayton, director of the Office of Research on Women’s Health at the NIH, is taking steps to ensure the inclusion of females even earlier in developing research.

As part of a new milestone, NIH grant applications are now evaluated on how they account for sex as a biological variable (SABV) in basic and preclinical studies with vertebrate animals and humans.

“Accounting for SABV in biomedical research will produce robust and relevant new discoveries about basic biology and help to inform sex- and gender-appropriate individualized health care for women and men, boys and girls,” wrote Clayton.

The new requirement is a progression from a commentary written by Clayton and Francis Collins, director of the NIH, published in the journal Nature in 2014. The article stressed the importance of analyzing data from female test subjects at early stages so that researchers are better informed when advancing to later phase trials.

The commentary offered a variety of examples of how drugs and diseases affect the sexes differently.

“We know much more about the role of sex and gender in medicine, such as that low-dose aspirin has different preventive effects in women and men, and that drugs such as zolpidem, used to treat insomnia, require different dosing in women and men,” wrote the authors in the commentary.

As a result of the commentary, the NIH began directing researchers to perform preclinical experiments with both sexes of animals, including those that use cell and tissue samples.

The latest announcement takes the policy a step further, by making a researcher's evaluation of SABV a major factor when considering grant approvals.  

“By asking researchers to take sex into account when designing studies and evaluating and reporting results, NIH will ensure that the influence of sex is examined across the spectrum of biomedical research. Appropriate consideration of the influence of sex in basic, preclinical and translational research contributes to NIH’s commitment to rigor and reproducibility in research, which will lead to a stronger foundation on which to build clinical research and trials,” said Clayton.