It's a real thing. Freakonomics talked about it in one of their recent podcasts. Basically, clinical trials often exclude or under sample women for a number of reasons including the confounding factors of menstrual cycles, pregnancy complications, etc. This has received increasing attention lately and is getting better, but a ton of the current scientific wisdom is based on underrepresentative samples.

http://freakonomics.com/podcast/bad-medicine-part-2-drug-tri...