Viagra was originally trialled as a drug for heart related health problems, when it went into human trials the subjects reported increased bloodflow elsewhere and as a result the use was changed to deal with erectile dysfunction.
interestingly they are now looking at it again as a heart disease drug.
IIRC, it wasn't that patients reported increased blodflow down there, but that a unusually large number of patients didn't return the drugs after the trial ended. So the researchers asked around, and learned that they trial patients didn't want to give up this "bonus".
interestingly they are now looking at it again as a heart disease drug.