It's absolutely a thing. Rampant in SV from what I hear. Many people get them prescribed (Adderall, Ritalin) for ADD, but it seems like 20-40% of programmers have diagnosed ADD. Others buy from friends who get it prescribed, or on the black market, and they probably don't talk about it at work.

I don't think most employers would fire an employee for taking drugs to improve their performance.

Regardless of safety (and I say this as someone who used to take prescription Adderrall myself) there are negative side effects, and long-term effects.