Quote Originally Posted by Bob the Awesome
What is HPPD?
From Wikipedia:

Hallucinogen persisting perception disorder (HPPD), is a temporary recurrence of disturbances in perception that are reminiscent to those experienced during one or more earlier intoxications with hallucinogens (often called Flashbacks).

But, this flashbacks are only a possibility. Probably there is a lot of people who did use a lot of hallucinogens and never had any.