In my experience...
...it builds up in you a bit. The first hit might not do anything, but the next one will hit you like a sledgehammer to whatever part of your brain makes you appreciate reality for what it is.

What I'm saying is don't expect to spend all day on it, and don't do too much more once you've had your initial hit. It'll leave you a gibbering wreck and you'll come to with most of a cigarette burnt away twixt your fingers. *ahem*

Otherwise, play safe and have fun! :D

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