A drug, typically amphetamine and especially one prescribed for medical use, that acts as a stimulant. They were originally advertised as pep pills to give people more energy in the '60s, then they were rebranded as diet pills in the '70s and '80s, before being taken off the market in the early 1990s. I was starting to wonder if my mother had gotten hooked on pep pills, judging by the way she'd been so jumpy and erratic lately.