After he finally "came down" from his bad LSD trip, M&M worried about having a flashback, which is a return of the hallucinations he experienced as a result of taking the drug.  If a flashback should occur, the effect could be just as severe as it was when M&M was actually under the influence of LSD the first time.

LSD is an illegal drug which was popular in the sixties.  It is a hallucinogen, and often had violent and permanently damaging effects.  In M&M's case, when he took the drug, he had vivid hallucinations about spiders.  He said that he "went into (his) stomach...down into (his) stomach and all these spiders came out...(he believed he) was there then years, and all that time (those) spiders kept chewing on (him)".  Before he was completely coherent again, M&M remembered screaming and screaming for help, but nobody came.  He said he "kept trying to get back, but the spiders held (him) down...and chewed on (him) and the colors went in and out...and they were screaming too".  In the midst of the terror of his bad trip, M&M tried to kill himself by jumping out the window; he was unsuccessful only because his companions took turns holding him down (Chapter 9).

M&M had been a sweet, intelligent, innocent kid, with an insatiable curiosity about the world, but the LSD trip changed him forever.  Bryon sees him at the drugstore several months later, and notices that M&M's "old expression of complete trust and intent interest is gone entirely".  M&M cannot remember things that happened in the past very well, and he tells Bryon that "(his) grades are shot"; as a young teen, his brain has been damaged forever.  M&M is nervous and constantly frightened.  He says to Bryon, "it can come back...I could have a flashback...and if I ever have any kids - something about chromosomes - they could be messed up" (Chapter 11).