In other games I've had players under the influence before. The approach depends on how much you can rely on them to keep player information separate and how much you need to compartmentalize the information for them.
This is a good situation because the drug is also allowing them to perceive the "monsters," right, so if you tell them they see a monster, is it a real one or just them tripping about a normal dog?
If you are not compartmentalizing information, you can just tell people the different things they perceive. But then it tends to just become "color;" players will occasionally act on the false info but tend not to when it actually matters. If you are just looking for scares, just exercise your powers of description in combat to the tripping characters. When they come to the stairs down, tell them "a huge hundred foot deep chasm opens up below you!" That kind of thing.
If compartmentalizing the information, use passed notes. Have the notes sometimes be the "real" perception of the situation and sometimes the hallucinations. Remember hallucinations aren't all visual "eek there's a monster," there's auditory stuff, enhanced visual (the colors are running!), etc. Did that one party member just make their check to hear something, or is it the drugs? And being in a suggestive state, it's reasonable for two people to say they thought they heard the same thing even if they didn't.
It is even easier if for whatever psychic-whatnot reason they do share the same hallucinations.
Mechanic-wise, I'd use Stability tests to indicate if someone is experiencing a hallucination at any given point (and I'd tend to do it myself behind the screen, because it somewhat defeats the point if they know who is tripping - but then again I'm not a big fan of narrativist play). Much like the mental stability rules, maybe let other non-tripping PCs try to corral them at times.