LSD

Lysergic acid diethylamide, abbreviated LSD or LSD-25, also known as lysergide (INN) and colloquially as acid, is a psychedelic drug of the ergoline family, well known for its psychological effects, which can include altered thinking processes, closed- and open-eye visuals, synesthesia, an altered sense of time and spiritual experiences, as well as for its key role in 1960s counterculture. It is used mainly as an entheogen, recreational drug, and as an agent in psychedelic therapy. LSD is non-addictive, is not known to cause brain damage, and has extremely low toxicity relative to dose. However, acute adverse psychiatric reactions such as anxiety, paranoia, and delusions are possible

LSD in "The Fillmore Shoggoth"
Prior to performing at the Fillmore, the rock band HPL dropped LSD provided to them by the legendary Kid Charlemagne, and then went out to perform. (On the way out to the stage, HPL's de facto leader, George had to navigate around the mirage of a pit full of fire; a mirage created by the LSD.)

The band was only part way through their second song when the house emcee announced that San Francisco was under attack by shoggoths The Fillmore began to evacuate, but then a shoggoth arrived. After causing substantial damage, the shoggoth was destroyed by napalm, and then the band evacuated to nearby Lafayette Park.

After eating and sleeping through the night, George awoke the next morning, and was able to listen to a radio report about the shoggoth attack. He learned that 16 people were known to have been killed at the Fillmore, that two shoggoths attacked the De Young Museum, destroying all the extensive collection of Old Ones artifacts, and that one shoggoth had attacked the zoo and killed all of the cave penguins. George found the deaths of the harmless penguins sadder than any other part of the attack, and concluded he may not yet have come down from his LSD high.