You know, this reminds me of a Neil Gaiman short story, "Changes" (for some reason I tend to pronounce it as Ch-ch-ch-ch-changes" in my head though) where a scientist finds a cure for cancer that has the side effect of changing the users sex. Generally the use of it for non-medicinal purposes is banned, which means that in black market it outsells heroin and cocaine. Has also side-effects such as it being forcibly being used on boys to increase their worth as prostitutes or being used in China to change the gender of newborn girls to boys. Despite this, still a generally rather positive story, I think.