Do: Know the actual, known effects of the drug you're writing about. Research as much as you can. Look for descriptions of other people's experiences.
Don't: Assume that because something is "a drug" that it has some extreme, trippy effect, a-la Mason Verger's experience of poppers in Hannibal.
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Remember the same drug can effect different people different ways. Cannabis might make one person giggle for an age, another pass out, and a third might not notice any effect at all right up to the moment they start throwing up.
Not everyone enjoys the experience they have on a given drug, even if the experience itself doesn't involve a "bad trip". And not everyone having a bad trip reacts badly.
Some people, especially more experienced people, might be aware that they are having a bad trip and try to deal with it. Someone I know once tried to play it cool and hide the fact that they were tripping their balls off while experiencing bleeding walls and spiders. In a pub. Which mostly translated to them trying to pick up a drink very, very slowly and ineptly while doing the drunk person's sober face.
No one says "heavy" any more. No matter how much of a huge stoner they are. Do research appropriate slang and the like for the time period you're writing, and tailor use of it according to the character.