By Elizabeth Cohen, CNN
Viagra has been on the market for years, but yet there's still no pill for women.
For 15 years Viagra has been a champion for its creator, Pfizer, the little blue pull earned $2 billion last year.
Now there's a whole subgenre of Viagra copycat drugs.
It's been so successful, the movie Love and Other Drugs immortalized the Viagra salesman.
So if sex drugs for men are so profitable, why isn't anyone selling them to women?
Women did try Viagra, by the thousands in clinical trials in the early 2000s, but it didn't work very well.
That didn't stop other pharmaceutical companies from chasing chemical romance.
A glob of testosterone was supposed to give a dose of desire, but studies show Libigel didn't really get libidos going. The company that made it never even brought it to the FDA.
Another drug, Flibanserin, did make it to the FDA, which rejected it, saying it had too many side effects and really didn't turn women on anyhow.
That left pharmaceutical companies asking, 'Why are women so hard to please?'
"The most common sexual problem for women is low desire. And that's very complicated because really desire is in our brains," said clinical psychologist Sheryl Kingsberg of Case Western Reserve University.
That's why this Stanford researcher is delving into the female brain, to see what makes it tick.
"You can see here this is a difference between women with low libido in the blue. And the yellow is women with normal libido," said Dr. Leah Millheiser of the Stanford University Female Sexual Medicine program.
Which may mean, that for women, the cure for low libido is more likely to be found in their brains, than in a bottle.