Slang for the perscription drugs Percocet or Percodan, which is an opiate or painkiller, commonly used recreationally for its euphoric effect.
30's are specifically Percocet
30 mg.
Ironically, Percocet is an opiate, which is a nervous
system depressant, and the slang term perc cannot be verbally distinguished from perk, which means to make more alert or active.