Apparently, it's very common for only male animals to be used in experiments. I can't believe no one ever thought this was a problem before. Sexism really runs this deep, guys.

Article

"Researchers avoided using female animals for fear that their reproductive cycles and hormone fluctuations would confound the results of delicately calibrated experiments."

And no one ever thought - hey, maybe human women will take these drugs eventually, and they have hormone fluctuations too! Maybe we should make sure these drugs still work on them!