[jonathankoren]

That wouldn't happen. You would simply declare, that you want to check if the drug does cause hair growth, and then run a test explicitly looking for that.

Unless you're looking for the effect from the outset, you can't be sure that what you saw wasn't actually random. There's a term for what you're describing, it's p-hacking, and it's explicitly the very thing declaring what you're looking for before you run the test is designed to prevent.

Like many science related topics, there's a XKCD about p-hacking that describes a similar scenario to your example: https://xkcd.com/882/