Why women were left out of clinical trials, and what changed

For decades, the people who took part in early drug trials were mostly men. The rules that shaped that came from specific decisions, and they were changed on specific dates. This article walks through the timeline and what the research says about the gap that remains.

The 1977 FDA guideline

In 1977 the FDA issued guidance recommending that women of childbearing potential be left out of early-phase (Phase 1 and early Phase 2) drug trials. It was a guideline, not a law, and it did not name all women. One review of the period describes the result plainly: in practice, this resulted in the exclusion of women.

1993: the FDA reverses, and Congress steps in

In 1993 the FDA replaced the guideline and removed the restrictions on women in Phase 1 and Phase 2 trials. The same year, Congress passed the NIH Revitalization Act (Public Law 103-43). It requires NIH-funded clinical research to include women as subjects, unless inclusion is inappropriate for the health of the subjects or the purpose of the research.

The NIH had encouraged inclusion since 1986, but the 1993 law made it a requirement. It applies to NIH-funded research, not to every company-run trial. That is 16 years from the 1977 guideline to the 1993 law.

The same bias in animal research

The gap did not stop with people. A 2011 review of 10 biological fields found male bias in 8 of them. In neuroscience, studies using only male animals outnumbered female-only studies 5.5 to 1.

The NIH policy on sex as a biological variable took effect on January 25, 2016. It expects researchers to factor sex into the design, analysis and reporting of vertebrate animal and human studies. It does not flatly require female animals in every study.

What the numbers look like now

A 2021 analysis of US clinical trials between 2000 and 2020 found women under-enrolled relative to disease burden in several fields. In oncology, women were 46% of the disease burden and 43% of participants. In neurology, the figures were 56% and 53%.

Those gaps are small, and male participants were under-represented in 8 other disease categories. A 2026 study in Nature Communications found female participation matched or exceeded disease prevalence in 67% of cases, with cardiovascular and autoimmune conditions the most under-enrolled. The picture has improved, and it is uneven.

What this does not tell us

• Enrollment share is not the same as results broken down by sex. In one World Economic Forum and McKinsey analysis, only 17% of heart disease trials completed in 2022 and open to both sexes published results broken down by sex.
• The 1977 guideline text itself could not be re-read for this article, so its wording rests on later reviews.
• Nothing here is medical advice about any medicine you take.

Frequently asked questions

Were women banned from clinical trials?

Not by law. A 1977 FDA guideline recommended leaving women of childbearing potential out of early-phase trials, and in practice that often meant exclusion. The FDA reversed it in 1993.

What is the NIH Revitalization Act?

It is a 1993 law (Public Law 103-43) that requires NIH-funded clinical research to include women, unless inclusion is inappropriate for the health of the subjects or the purpose of the research.

Are women still under-represented in trials?

In some fields, yes, by small margins. A 2021 analysis found women under-enrolled relative to disease burden in oncology, neurology, immunology and nephrology. A 2026 study found female participation matched or exceeded disease prevalence in 67% of cases.

Related reading

• Do women need different drug doses? What the research says
• Women's health research funding: the gap in numbers
• Gender gaps in care: from the emergency room to heart procedures
• How women entered medicine, and what doctors are still taught
• The women's health research gap: what the evidence shows
• Why women's health is decades behind: the research gap
• Dosed like a smaller man: why women get the wrong drug doses
• Why medicine is not one-size-fits-all for men and women

Sources

1. AAMC, Why we know so little about women's health (2024).
2. Reviews describing the 1977 FDA guideline "General Considerations for the Clinical Evaluation of Drugs": Frontiers review, 2026 and review, 2014.
3. NIH Revitalization Act of 1993, Public Law 103-43, Section 131 (Section 492B of the Public Health Service Act). FDA, Study and Evaluation of Gender Differences in the Clinical Evaluation of Drugs (1993).
4. Beery AK, Zucker I. Sex bias in neuroscience and biomedical research. Neuroscience and Biobehavioral Reviews, 2011 (PMID 20620164).
5. NIH policy on sex as a biological variable, notice NOT-OD-15-102, effective January 25, 2016. See NIH Office of Research on Women's Health authors, 2022.
6. Steinberg JR et al. Analysis of female enrollment and participant sex by burden of disease in US clinical trials between 2000 and 2020. JAMA Network Open, 2021.
7. Sex representation in trials relative to indication-specific disease burden in FDA-approved drugs, 2015 to 2023. Nature Communications, 2026.
8. World Economic Forum with McKinsey Health Institute, Blueprint to Close the Women's Health Gap (2025).

This article is for general education and is not medical advice. Sources were checked in October 2026. Several figures come from secondary analyses, as noted above.