Women are badly underrepresented in health research, including sport and exercise studies and even drug trials, and a big reason for the potentially dangerous disparity is their monthly period, experts say.

An editorial in the British Journal of Sports Medicine looked at the gender gap in sports and exercise research and found menstruation is the major barrier.

"Females undergo a variation in hormone levels through the menstrual cycle. So as a result of this, they appear to be more complex," said lead author Georgie Bruinvels, a PhD student at University College London in the U.K.

She points to a review in 2014 of more than 1,300 sport and exercise research studies involving more than six million participants. On average, 39 per cent of the participants were women and 61 per cent were men.

There was a time when women were left out of research because there were concerns drug trials might harm unborn babies. Researchers also didn't want to risk including women, the editorial says, because they were "more physiologically variable than men."

Since men were viewed as adequate proxies for women, years of being left out of important research was considered insignificant, the editorial says.

'Not scientifically correct. Period. Full stop'

Dr. Jerilynn Prior, a professor of endocrinology at the University of British Columbia, says men are not adequate replacements for women in research.

"Not scientifically correct. Period. Full stop," she told CBC News.

Excluding women from drug trials creates an imbalance, she says. Millions of women and men are prescribed the same drugs every day. Yet some of those drugs were tested only on men.

"Half the human race doesn't have accurate information about the response to whatever the intervention is," Prior said. "It's like comparing apples and oranges."

The U.K.'s Heather Watson at Wimbledon in 2015. Earlier the same year, she pulled out of the Australian Open because of "girl things." An editorial in the British Journal of Sports Medicine looked at the gender gap in sports and exercise research and found menstruation is the major barrier. (Kirsty Wigglesworth/Associated Press)

This disparity exists, she says, even though a policy of the Canadian Institutes of Health Research requires researchers to say how they are dealing with sex and gender when applying for research grants.

"Rarely ever are half of the participants in trials women," said Prior, the founder and scientific director of the Centre for Menstrual Cycle and Ovulation Research.

"So how can you apply information that relates only to men, to women whose physiology is different?"

Bruinvels says there are dangerous consequences to that imbalance because women respond differently to drugs.

80% of drugs withdrawn from market

"Evidence actually suggests that women are almost twice as likely to have an adverse reaction to a drug than a male counterpart," she said.

In fact, a U.S. accountability study found "80 per cent of drugs there are withdrawn from the market due to unacceptable side-effects on women."

"There are metabolic differences," Prior said. "How the drug is complexed [released] or excreted may vary a lot in women versus men. So then you have new risk factors that you didn't even know were a possible risk."

Women not just men with 'boobs and tubes'

In 2013, the U.S. Food and Drug Administration released a safety announcement about the sleep aid, zolpidem, also known as Ambien. It recommended the bedtime dose be lowered for men and women. It also warned that women are more susceptible to risks associated with the medication because they metabolize the drug at a slower rate than men.

"Women are not just men with boobs and tubes," Dr. Alyson McGregor of Rhode Island said during a recent TED Talk on women's health. "They have their own anatomy and physiology that deserves to be studied with the same intensity."

Georgie Bruinvels is currently researching how iron deficiency in menstruating women impacts their sports performance. The menstrual cycle can be a barrier for women in sport. Last year, for example, British tennis player Heather Watson blamed "girl things" when she pulled out of the Australian Open.

Bruinvels is trying to break down the barrier for women in research. She's hoping to foster a greater understanding of the menstrual cycle and to address the ongoing disparity in not only sport and exercise research, but all types of medical research.

"Until we have that," she told CBC News, "there's just going to be this kind of grey cloud over the female."