Why are clinical trials still dominated by men? Gender gap raises concern

A clinical trial participant presses their arm after getting blood drawn while leaving a hospital in Seoul’s Gangseo District, Sept. 26. Korea Times photo by Choi Joo-yeon

A line of people rolled their suitcases out of a clinical trial center in Seoul’s Gangseo District on Oct. 18. Leading the group was a young man wearing a black cap pulled low. Several men in hoodies and athletic wear followed. By the time the last participant walked out, 50 people had exited the building. All of them were men.

According to the Ministry of Food and Drug Safety, clinical trial participants last year included about 92,600 men and about 68,600 women, showing a clear gap of more than 30 percent.

The pattern was the same in previous years. In 2022, about 90,000 men participated compared with about 65,500 women. In 2023, about 89,500 men participated compared with about 67,100 women.

An investigation by the Hankook Ilbo found similar results. Reporters visited about 20 hospitals conducting clinical trials for healthy volunteers more than 20 times in August and observed that most centers were filled with male participants. This was true even when recruitment postings said “healthy men and women aged 18 or older.”

People with previous experience in trials said the same. Although some studies accept women, the slots are fewer and often close quickly. In some cases, women apply but are not selected.

“If there are 10 postings, eight are for men only and two include women. The quota for women is smaller, so it fills fast," said Kim Hye-min, 31, who joined a diabetes treatment trial. In her most recent trial, she said there were eight women and 32 men.

Why women are screened out

Industry officials say the imbalance comes from how the system operates. Pharmaceutical companies and hospitals want trials with no disruptions. These early-stage studies depend on collecting a consistent number of blood samples at exact times. If a participant drops out, the schedule collapses and the entire study may be halted.

For this reason, companies ask recruitment platforms to secure participants with as few variables as possible, in large numbers and within a short period. Young men are seen as the group with the least hormonal fluctuation.

An official from a major recruitment platform said women face higher risk of exclusion because “menstruation, pregnancy and hormonal changes can distort drug-response data, which increases the chance of mid-trial dropout.”

From the recruiters’ point of view, experienced participants are also easier to manage, and repeat volunteers are overwhelmingly men. Even for trials of treatments for diseases more common in women, such as dementia or hypotension, male participants often make up most of the wards.

A staff member at a contract research organization that connects drugmakers and hospitals said companies “take the easiest path” when they are not required to recruit specific patient groups. A recruiter added, “Drugmakers want healthy volunteer trials to proceed without delays.”

Male participants leave a clinical trial center in Seoul after completing an inpatient stay in September. Korea Times photo by Choi Joo-yeon

Male-centered trials create health inequities

The dominance of male participants can distort results. When women’s physiological patterns are underrepresented, sex-specific side effects may be missed.

A well-known example is the sleep aid zolpidem. The drug produced stronger side effects in women, such as drowsiness while driving and difficulty concentrating the next day.

Later research showed that because its fat-soluble components remain longer in bodies with higher fat content, women were more affected. The U.S. Food and Drug Administration reduced the recommended initial dose for women to half that of men in January 2013.

A domestic case also raised questions. A Korean irritable bowel syndrome medication was found by patients to cause constipation in women at standard doses.

A 2023 Seoul National University study said that when factors such as age, sex, race and health condition influence drug metabolism, “clinical trials that do not reflect this can produce inaccurate data.”

Because of such risks, many countries require gender diversity in trials. The United States passed a law in 1993 mandating the inclusion of women, and U.K. regulators also emphasize diversity plans that include gender balance.

Experts in Korea say similar measures are needed. Kim Na-young, director of the Sex Differences Research Institute at Seoul National University Bundang Hospital, said both new and generic drug trials should ensure that “no single sex drops below one third of total participants” as part of a pilot regulation.

This article from the Hankook Ilbo, the sister publication of The Korea Times, is translated by generative AI and edited by The Korea Times.