It's a Roofie: Well if you're a hot chick I would recommend never taking a pill from any creepy looking guy, no matter how much they stress that, "It's certainly not a roofie." 9 times out of 10 this guy is lying.
Guys on the other hand, yea you may die from a roofie, but if this chick is hot, I mean, and she's trying to rape you; that would be an awesome story and in my opinion: worth it.

It's Tamiflu: Swine Flu is a big concern lately and who knows? Maybe she's a pharmacist and she walks around raves treating people. Now, this pill is more commonly in a pill shape and has yellow on half of it, while ecstasy more often then not has a picture of something you'd see on a pair of Claire's earrings.

It's Birth Control: Well, for starters, that chick is probably a man; and you either are a woman or look frighteningly so like one. Either way that man/chick is a weirdo, so 9 out of 10 cases of this happening the best thing you can do is to just run far away.