There are several verbs. The verbs "to poison" and "to drug" exist and could easily be used in this context. There is also a verb used in informal speech - "to spike" - that can refer to either drugs or alcohol. (Definition # 5 here.)
You can say "Somebody drugged my drink". That's probably the most common. It's also possible to say "Somebody spiked my drink with something", or "Somebody slipped me a mickey". The last one sounds really old-fashioned to me.