I used LSD I reckon about 125-150 times over two or three year period and have never had a problem with flashbacks. Flashbacks, incidentally, don't exist. I suspect the idea was created to strike fear into the minds of naive and impressionable youth of the 60s. Fortunately it didn't work.
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"All my years in p*rn didn't quite prepare me for childbirth. I mistakenly thought all the stretching I did would make this easier."