The female data gap in health research

Key takeaways

• From 1977 to 1993, FDA guidance advised against women who could become pregnant joining the earliest drug trials.
• Since the NIH Revitalization Act of 1993, US law has required the NIH to include women in the clinical research it funds.
• A 2001 government report found that 8 of 10 drugs recently withdrawn from the US market posed greater health risks for women.
• Women have adverse drug reactions nearly twice as often as men, and many drugs reach higher levels in women's blood.
• Inclusion is improving, but analysis by sex still lags behind.

The female data gap at a glance

What is the female data gap?

The female data gap is the difference between how much research has been done on male bodies and on female bodies. It comes from women being left out of studies, and from studies that include women but never look at their results separately.

It matters because women and men can process the same medicine differently. When studies skip that, women can get doses and advice tested mostly on men.

Why were women left out of drug trials?

Mainly to protect possible pregnancies. A 1977 FDA guideline, written in the shadow of the thalidomide tragedy, said women of childbearing potential should not be included in phase 1 and early phase 2 drug trials.¹

Women could join later trials once animal studies on fertility and birth defects were done and the drug showed some benefit. There was an exception for life-threatening diseases, though the FDA later noted that women were sometimes excluded even then.¹

Early trials are where doses and safety are first tested in people. Leaving women out at that stage meant much of the first dosing data came from men.

When did women have to be included in clinical research?

In 1993. In June, the NIH Revitalization Act became law and told the NIH to make sure women are included in the clinical research it funds.² In July, the FDA revoked the 1977 restriction on women joining early drug trials.¹

The law also asked that large phase III trials be designed so researchers can tell whether a treatment affects women differently.² The FDA's 1993 guideline did not require women in any particular trial. It left the choice to patients, investigators, drug sponsors and ethics boards.¹

Do drugs affect women differently?

Often, yes. A 2001 report by the US General Accounting Office found that 8 of the 10 prescription drugs withdrawn from the US market since January 1997 posed greater health risks for women than for men.³

In four cases, this may have been because women were prescribed the drugs more often. In the other four, women had more harmful side effects even though the drugs were widely prescribed to both sexes.³

A 2020 review found that women experience adverse drug reactions nearly twice as often as men. Of 86 drugs studied, 76 showed higher drug levels or slower clearance in women. For drugs with clear side effects, these differences predicted which sex had more reactions in 88% of cases, and body weight did not explain it.⁴

One example is the sleep drug zolpidem. In 2013, the FDA lowered the recommended dose for women from 10 mg to 5 mg for immediate-release products, because women clear it more slowly and appear more at risk of next-morning impairment, including when driving.⁵

How many sports science studies include women?

Few study women alone. A review of 5,261 studies published from 2014 to 2020 in six leading sport and exercise science journals found that 63% included both sexes, 31% included men only and just 6% included women only.⁶

Of more than 12.5 million participants, 34% were female. The authors concluded that most findings in the field might only apply to one sex.⁶

When a training plan says research shows something, it is fair to ask who was studied.

Is the female data gap closing?

Slowly. In 2016, the NIH began expecting sex as a biological variable to be factored into the design, analysis and reporting of the animal and human studies it funds. Under the policy, applications to study only one sex must give strong justification.⁷

The NIH explained that while women made up roughly half of participants in its clinical research, basic and preclinical studies had more often focused on male animals and cells.⁷

Progress is real but incomplete. A 2026 study of 574 NIH-funded papers from 2017 to 2024 found that 61% included both sexes. Among those, 83% reported how many of each sex took part, yet only 44% ran an analysis by sex.⁸

Why does Regime Longevity start from the female body?

Because a woman's month is not one flat line. Hormones rise and fall across the menstrual cycle, and much everyday health advice was never built around that.

Regime Longevity predicts your period and your phase, then builds a daily plan for food, training, sleep and work that fits the phase you are in. You can add your blood test results, and an AI concierge can change the plan when your day changes.

To be clear about the limits: an app cannot close a research gap, and Regime is a wellness app, not a medical device. Research on the cycle, training and food is still growing, so the plan follows what you log and how you feel.

Keep reading