Believed to be Marijuana. But say, if marijuana did not exist, wouldn't that mean people would go directly to harder drugs? So say "if" marijuana is a gateway drug, that's not such a bad thing...
In a world without marijuana (Gateway drug)
Person 1: Hey man, wanna shoot some smack?
Person 2: Of course, it's too bad there isn't some magical herb we could smoke instead that slows people like us from getting to the real dangerous shit!!!
Person 1 & 2 shoot up.