PCP: general

PCP (phencyclidine) is a synthetic dissociative anesthetic that people use recreationally for its hallucinogenic effects [1][2][3]. It acts on the central nervous system and changes mood, behavior, and perception [4]. It's sold illegally as a white powder, and also turns up as an oil, liquid, pill, or crystal [4][5][3].

The two things to know before anything else:

It can make you dangerous to yourself or others while you're on it. People intoxicated on PCP may become violent or suicidal, which puts both the user and people around them at risk [6].

High doses can kill. Seizures, coma, and death are documented at high doses [6]. As with dissociatives generally, overdose can range from unpleasant to life-threatening depending on dose, route, and other substances on board [7]. As with dissociatives generally, signs of overdose include severe nystagmus (eye twitching), complete unresponsiveness, vomiting while unconscious (aspiration risk), severely depressed breathing, seizures, and extremely elevated heart rate or blood pressure [7]. Any of these is an emergency.

Mixing matters. Because PCP can be sedating, combining it with other depressants — alcohol, benzodiazepines — can lead to coma [6]. As with dissociatives generally, this class of drug can also cause respiratory depression, heart rate abnormalities, and a withdrawal syndrome [8].

If you're going ahead: don't use alone, and don't combine with other depressants.

References

1. Phencyclidine — U.S. Drug Enforcement Administration
2. Phencyclidine Toxicity — National Center for Biotechnology Information
3. PCP - Canada.ca — Health Canada
4. Substance use - phencyclidine (PCP): MedlinePlus Medical Encyclopedia — U.S. National Library of Medicine
5. Phencyclidine (PCP) — Drug Science (UK)
6. Dissociative Drugs — New York City Department of Health and Mental Hygiene
7. Dissociatives: Dosage, Effects, Duration & Safety Guide — SubstanceWiki
8. Hallucinogens and Dissociatives — National Institute on Drug Abuse (NIDA)