How it affects you will really depend on the amount of each that you're taking. Generally though, no--a combination of LSD/acid and Xanax is not going to put you in a coma-like state. But some people do find that even small doses of Xanax or other benzodiazepines make them feel slow to respond or very sleepy. Which is very different from a coma, of course.
It's actually pretty common for people who use LSD to take Xanax as well. Some people take it as a way to stop the trip and others take it to kind of mellow the trip and make it less intense (since Xanax is an anti-anxiety drug and LSD trips can be unpredictable and sometimes panic-inducing). I can't say I think that's a particularly good idea, but as far as I know it's not a toxic combination.
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