500 words? Thats a fucking joke, come on dude. Theres so many things you could easily write about given such a short speech.
Perhaps instead of "drugs" as most people would think (coke, heroin, meth etc.), talk about caffiene or alcohol or something. Maybe go with the idea that someone else mentioned about how 100% of peopple who drink coffee will die or something like that.
Or, make your whole speech as if you were constantly tripping balls. Talk about things people would think/see on hallucinogens as if they were common occurences in everyones life.