M-99 (or M99) is the shortened term for the tranquilizer known as Etorphine Hydrochloride. It is used commonly as an animal tranquilizer and is known to be 1000 times stronger than morphine. This drug is commonly used in the Showtime series Dexter.
Dexter Morgan: What am I even looking for?
Vince Masuka: Wait for it.
Dexter Morgan: This is a prelim. I don't have time to...
Vince Masuka: M-Fucking-99 (M-99)
Dexter Morgan: Etorphine Hydrochloride?
Vince Masuka: An animal tranquilizer more powerful than morphine. Causes total paralysis. That mark on her neck kept bugging me, so I ordered up a tox screen.
Dexter Morgan: (voice over) That's it. No more doughnuts for Masuka.