More Cancer Clinical Trials Include Women. But Inclusion Isn’t Enough.

A woman facing an innovative cancer treatment may ask her doctor, “Will this treatment affect me differently because I'm a woman?” Unfortunately, too often her doctor has to answer, “I don't know.”

Over the past decade, cancer research has made real progress getting more women into clinical trials for new cancer drugs. That's genuine progress. But counting women in a trial isn't the same as understanding how a treatment affects them differently than it affects men. Inclusion is not enough.

A Data Gap

Women and their doctors deserve to know what to expect in cancer care, but few trials explore whether a cancer treatment works differently, or causes different side effects, in women versus men. A study in Nature Communications reviewed more than 89,000 oncology trials and found that only 0.5% reported comparing outcomes by sex.¹

This oversight can have significant effects on women. A Journal of Clinical Oncology analysis spanning 30 years and more than 23,000 patients found that women had a 34% higher risk of severe adverse effects than men, and 49% higher among women receiving immunotherapy.²

My colleagues and I discussed this research gap in a recent article in Nature Reviews Urology.³ We found cancer trials are doing a better job of enrolling more women, but they are still not routinely studying how treatment affects women differently once they're enrolled.

Biological Differences

Men and women have biological differences that affect how drugs work in the body.⁴ Estrogen changes how some immunotherapies affect patients. Kidneys filter medication at different rates in women and men. Drugs also break down, or metabolize, differently depending on sex, which changes how long a drug stays in the body and how strongly it affects a patient.

When trials don't study these differences, women can be exposed to side effects that might otherwise have been predicted – and possibly reduced.

What Needs to Change

The FDA already offers guidance encouraging more inclusive trial design, addressing both biological and logistical barriers to enrolling women. That's a start, but it isn't enough on its own. No single hospital or research institution can fix this alone. Closing the gap will take clinicians, researchers, regulators, drug makers, and patient advocacy groups working together to design and report more well-rounded studies.

Bringing more women into cancer trials was an important first step. But patients and their physicians want to understand how treatment affects women, not just how many were enrolled. Getting there means we keep pushing until sex-based data isn't the exception in cancer research, but the standard.

David J. Benjamin, MD, is a board-certified medical oncologist at Hoag Family Cancer Institute.

REFERENCES

1. Kammula AV, Schäffer AA, Rajagopal PS, Kurzrock R, Ruppin E. Outcome differences by sex in oncology clinical trials. Nat Commun. 2024 Mar 23;15(1):2608. doi: 10.1038/s41467-024-46945-x

2. Unger JM, Vaidya R, Albain KS, LeBlanc M, Minasian LM, Gotay CC, Henry NL, Fisch MJ, Lee SM, Blanke CD, Hershman DL. Sex Differences in Risk of Severe Adverse Events in Patients Receiving Immunotherapy, Targeted Therapy, or Chemotherapy in Cancer Clinical Trials. J Clin Oncol. 2022 May 1;40(13):1474-1486. doi: 10.1200/JCO.21.02377

3. Benjamin DJ, Zaaijer S, Feinstein S, Rezazadeh Kalebasty A. Why counting women is not enough: from inclusion to equity in oncology trials. Nat Rev Urol. 2026 Jun 16. doi: 10.1038/s41585-026-01170-7

4. Study of Sex Differences in the Clinical Evaluation of Medical Products. US FDA. Accessed August 24, 2026. https://tinyurl.com/2357uzw6