I'm gueesing because when you're in a club it's loud and lots going on around you, less converstaion, much more visual stimulation... no real need to concentrate on things.
Most hallucinogens will give people that paranoid feeling. Acid used to make me alienated from anyone who didn't drop it with me. And pcp often gave me and my friends a feeling of doom like the police would be there at any minute, fun times nonetheless

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Quick answer: You're ****ing up your brain, that's why [xx(]