Some medical studies exclude participants based on their sexual orientation, say U.S. cancer researchers who urged scientists to explain their criteria.

The letter in Thursday's New England Journal of Medicine includes examples of 37 studies out of 243 searched or 15 per cent that had explicit exclusionary language, such as requiring participants to be "in a reciprocal relationship with a person of the opposite sex."

Some trials are restricted to heterosexual patients because of the nature of the study, such as how HIV spreads between heterosexual partners. But the researchers also found trials where there is no clear need to exclude lesbians and gay men, including a clinical trial of attention deficit-hyperactivity disorder.

Brian Egleston, a biostatistician at Fox Chase Cancer Center in Philadelphia, decided to gather data on clinical trials after overseeing enrolment of cancer patients. He noticed some explicitly excluded those in same-sex relationships.

"When I first saw this, I thought it was a fluke. The second time, I thought I'd dig deeper," Egleston said.

To find out more, Egleston and his colleagues searched through ClinicalTrials.gov, a database of more than 80,000 clinical trials sponsored by the U.S. National Institutes of Health, other government agencies and private industry.

The results suggest exclusion of lesbians and gay men from clinical trials in the United States is not uncommon, particularly in studies dealing with sexual function, the researchers said.

"It is likely that most gay and lesbian patients are unaware that their sexual orientation is being used as screening factor for participation in clinical trials," the authors wrote. "Researchers should be held to careful scientific reasoning when they develop exclusion criteria that are based on sexual orientation."

To apply for federal funds in the U.S., a study can't exclude based on gender, race or ethnicity.

Egleston said the exclusion bias from earlier trials on a similar subject may get repeated when researchers copy and paste participation criteria.

With files from The Associated Press