Post Hoc Fallacy

A Post hoc fallacy, also known as "coincidental correlation" or "false cause," is a logical fallacy which assumes or asserts that if one event happens after another, then the first must be the cause of the second. It is a particularly tempting error because temporal sequence is integral to causality â?? it is true that a cause always happens before its effect. The fallacy lies in coming to a conclusion based only on the order of events, which is not an accurate indicator. That is to say, it is not always true that the first event caused the second event.

Found that in a speech class textbook. I love how precisely that describes what the government does by telling people that marijuana is a gateway drug and providing as proof that a large number of people on harder drugs did in fact use marijuana before they began using harder drugs.