Phencyclidine

Phencyclidine (PCP), chemically 1-(1-phenylcyclohexyl)piperidine, is a synthetic dissociative NMDA receptor antagonist first synthesized in 1956 by Victor Maddox at the Parke-Davis Research Laboratories in Detroit, Michigan, during a systematic screening program for new general anesthetics. It is the prototype of the arylcyclohexylamine class, the chemical family that includes Ketamine, Methoxetamine, and the modern research-chemical analogs (3-MeO-PCP, 4-MeO-PCP).

Parke-Davis marketed phencyclidine briefly as Sernyl for human surgical anesthesia from 1963, then withdrew it from human use in 1965 after consistent reports of severe emergence reactions, prolonged psychosis, and a clinical presentation that anesthesiologists found unmanageable. Veterinary use as Sernylan (large animal anesthesia, particularly for primates) persisted until 1978. By that point, phencyclidine had become a notorious street substance under the names angel dust, dust, sherm, wet, and (misleadingly) embalming fluid, and the United States Drug Enforcement Administration had moved it from Schedule III (its original 1970 placement) to Schedule II.

Phencyclidine has no current medical indication anywhere in the world. Its enduring importance is scientific. The binding site within the NMDA receptor channel pore is still called "the PCP site". Krystal, Javitt and others established in the 1990s that NMDA antagonism in healthy volunteers reproduces both the positive AND the negative symptoms of Schizophrenia more faithfully than any Psychostimulant model, which underpins the glutamate hypothesis of schizophrenia and the entire generation of NMDA-targeted therapeutic candidates that followed. Ketamine's reframing as a rapid-acting antidepressant and the approval of dextromethorphan-bupropion (Auvelity) are downstream of work that began with phencyclidine.

Dissociative, NMDA receptor antagonist, Sigma-1 receptor agonist, Arylcyclohexylamine, Anesthetic (historical)

Phencyclidine

Sernyl (human, withdrawn 1965); Sernylan (veterinary, withdrawn 1978)

Experience

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Titration strategies

Oral dose ladder (Erowid)▲0▼×

Erowid's dosage documentation for oral PCP reports the following tiers (threshold

and heavy not characterized):

• Light: 3-5 mg
• Common: 5-10 mg
• Strong: 10+ mg

Duration approximately 4 to 6 hours with aftereffects up to 24 hours. Timing not specified by Erowid. PCP is frequently applied to plant material (cannabis or tobacco) and smoked; specific dose tiers for

smoked administration are not documented by Erowid.

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Summary

Classes

Dissociative, NMDA receptor antagonist, Sigma-1 receptor agonist, Arylcyclohexylamine, Anesthetic (historical)

Common uses

Historical: dissociative surgical anesthesia (withdrawn 1965)▲0▼, Research tool: NMDA receptor pharmacology and the glutamate hypothesis of schizophrenia▲0▼

Pharmacy

Starting dose

No current medical indication. Historical anesthetic dose 0.25 mg/kg IV (Sernyl).

Preparations

Historical: Sernyl 25 mg tablets, 10 mg/mL injection (human); Sernylan 100 mg/mL injection (veterinary). Illicit: white crystalline powder, oily liquid, "dipped" cigarettes ("wet"), tablets.

US FDA Max

N/A (no current medical indication)

Pharmacology

Routes

Oral, IV, IM, smoked, insufflated

Onset

Smoked 2-5 min; insufflated 5-15 min; oral 30-60 min; IV / IM ~5-15 min

Duration

4-8 hours typical; longer at high doses; residual cognitive and perceptual effects up to 48 hours

Half-life

Highly variable, 7-46 hours (mean ~21 h); lipophilic deposition in fat with delayed re-release contributes to wide range

Bioavailability

~72% oral; ~85% smoked

Pregnancy

Limited human data; case series describe neurobehavioral effects in exposed neonates. No medical indication exists; recreational use in pregnancy is associated with maternal hypertensive emergency and possible developmental sequelae.^{[citation needed]}

Legal status

Schedule II controlled substance in US (rescheduled from Schedule III in 1978). No accepted medical use. UN Convention on Psychotropic Substances Schedule II internationally.

Purported mechanism

Phencyclidine is a non-competitive open-channel blocker of the NMDA glutamate receptor, binding to a site within the channel pore (the "PCP site", named for this medicine) that is accessible only when the channel is open; the resulting use-dependent block produces dissociation, anesthesia, and the broad cognitive disruption characteristic of NMDA antagonism.▲0▼ Secondary activity at sigma-1 receptors (agonist), the dopamine transporter (weak inhibition), and nicotinic acetylcholine receptors (antagonist) shapes the distinctive clinical profile compared with the cleaner profile of ketamine. Metabolized by CYP3A4 and CYP2B6; the parent compound is the principal active species.

History

Synthesis and first human anesthetic trials, 1956-1962

The arylcyclohexylamine scaffold emerged at Parke-Davis in the mid-1950s as part of a search for analgesics and anesthetics free of the respiratory depression and circulatory instability of the opioids and barbiturates then in routine surgical use. Victor Maddox, working in the medicinal-chemistry group, synthesized phencyclidine in 1956; his colleague Graham Chen described its remarkable animal pharmacology over the next two years, including the unusual combination of profound analgesia and anesthesia without significant cardiovascular or respiratory depression, but with a state that did not look like ordinary sleep.

The first human anesthetic trial, by Frederick Greifenstein and colleagues at Wayne State University in 1958, reported what they called "a profound analgesia with a peculiar mental state". Patients appeared awake but disconnected: eyes open, sometimes tracking, but unresponsive to surgical stimulus and afterward amnestic. Edward Domino, who would become the field's chief historian, joined the project in the late 1950s and coined the term dissociative anesthesia to describe the state, distinguishing it from both classical general anesthesia and from the twilight sedation produced by sedative-hypnotics.

Sernyl approval and the emergence problem, 1963-1965

The US Food and Drug Administration approved phencyclidine for human surgical anesthesia in 1963, marketed by Parke-Davis as Sernyl. The initial enthusiasm was real: Sernyl produced surgical anesthesia with preserved airway reflexes, preserved cardiovascular tone, and no respiratory depression, in patients whose comorbidities made conventional general anesthesia hazardous. Anesthesiologists who used it described it as a near-ideal agent intraoperatively.

The problem was emergence. In the first published series of any size, 10 to 30% of patients emerged from Sernyl anesthesia with severe agitation, frank hallucinations, paranoid delusions, and a psychosis that in some cases persisted for days to weeks after a single anesthetic dose. Surgical recovery rooms became unmanageable. Reports accumulated of patients who recovered from anesthesia, were discharged, and presented to psychiatric emergency rooms in florid psychotic states a week later. Parke-Davis withdrew the human indication in 1965, four years after FDA approval, and refocused the program on a structural analog that the chemist Calvin Stevens had synthesized in 1962 and that produced shorter, less intense, and more medically manageable dissociation: ketamine. The decision to push ketamine and shelve phencyclidine for human medicine was, in retrospect, prescient: ketamine remains in routine clinical use globally six decades later.

Veterinary Sernylan and recreational emergence, 1967-1978

Phencyclidine continued in veterinary use as Sernylan for nearly a decade and a half after the human withdrawal, principally for large-animal and non-human-primate anesthesia where the preserved cardiovascular tone outweighed the emergence concerns that ended the human program. Sernylan was withdrawn in 1978.

Recreational use of phencyclidine emerged in San Francisco in 1967, where it appeared briefly as a tablet sold under the name "the PeaCePill"; the experience was widely reported as unpleasant, and phencyclidine nearly disappeared from the West Coast scene within a year. It returned in the early 1970s in a different form: as a powder, frequently smoked after being sprinkled on cannabis or parsley, or "dipped" into a solvent and applied to cigarettes ("wet", "fry", "sherm" - the last from the Sherman brand of cigarettes commonly used as the substrate). Liquid forms were sometimes sold or described as embalming fluid; some samples genuinely contained formaldehyde as a solvent for phencyclidine, while other samples sold under the name were formaldehyde alone with no phencyclidine, an interchangeability that confused both users and clinicians.

By the mid-1970s, urban emergency departments in the eastern United States were seeing enough acute phencyclidine intoxication to make it a routine differential in any agitated, hypertensive, dissociated patient of unclear etiology. The DEA rescheduled phencyclidine from Schedule III to Schedule II in 1978, citing the public-health burden of the dipped-cigarette epidemic.

The 1980s media moral panic

The 1980s brought the cultural moment that still shapes American lay perception of phencyclidine: a series of news and law-enforcement narratives in which intoxicated users displayed apparently superhuman strength, indifference to injury, and resistance to physical restraint. The accounts were exaggerated in their cinematic details but had a real toxicologic basis. The combination of profound dissociation, analgesia, motor disinhibition, and adrenergic surge can produce behavior in which a user breaks bones, dislocates joints, or sustains serious injury without subjectively perceiving it, and can produce muscular performance unconstrained by the normal sensory feedback that limits exertion. Cases of self-injury, of fractures discovered in custody after a phencyclidine-intoxicated arrestee was restrained, and of post-arrest Rhabdomyolysis are documented in the toxicology literature of the period. The mythology layered on top of these real phenomena was substantially overstated; the underlying pharmacology was not invented.

Scientific exploitation begins, 1979-1994

In parallel with the recreational epidemic, basic researchers were using phencyclidine to dissect a question that conventional pharmacology had not been able to answer: which receptor system, when disrupted, produces something that looks like Schizophrenia? The dominant model in the 1970s was the dopamine hypothesis, based on the neuroleptic activity of D2 antagonists and the psychotogenic activity of Amphetamine and other psychostimulants. The psychostimulant model reproduced positive symptoms (delusions, hallucinations, paranoia) but failed to reproduce negative symptoms (affective flattening, alogia, avolition) or the cognitive deficits.

Nick Anis, Susan Berry, Nigel Burton and David Lodge at the University of Bristol showed in 1983 that phencyclidine and ketamine selectively reduce excitation of central neurones by N-methyl-D-aspartate, identifying for the first time that the molecular target of dissociative anesthesia is the NMDA glutamate receptor. Daniel Javitt and Stephen Zukin at the Albert Einstein College of Medicine consolidated the clinical implications in their 1991 review, articulating the NMDA hypofunction hypothesis of schizophrenia: that phencyclidine reproduces the full symptom complex of schizophrenia (positive, negative, AND cognitive) better than any other pharmacologic model precisely because it blocks NMDA-mediated glutamate signaling, and that endogenous NMDA hypofunction may underlie the disorder. John Krystal and colleagues at Yale tested this directly in 1994 by administering subanesthetic ketamine to healthy volunteers, who developed transient positive-symptom-like perceptual disturbances and (importantly) the cognitive and negative-symptom-like features that the psychostimulant model could not produce.

This work redirected schizophrenia neuroscience for the next generation and made phencyclidine, even after its medical withdrawal and its descent into the street, one of the most scientifically productive molecules of the late twentieth century.

Pharmacology

Receptor activity

Phencyclidine's principal action is non-competitive open-channel block of the NMDA receptor. The NMDA receptor is a glutamate-gated cation channel essential for excitatory synaptic transmission, synaptic plasticity, and learning. Phencyclidine binds at a site deep within the channel pore (the PCP site) that is sterically accessible only when the channel has been opened by glutamate binding and membrane depolarization; this gives the block its use-dependence, meaning that more-active synapses experience disproportionately greater inhibition. The PCP site is shared by Ketamine, Dextromethorphan, Memantine, Methoxetamine, and the experimental tool compound MK-801 (dizocilpine), although the binding affinities and the kinetics of unblock vary widely across the class and account for much of the difference in their clinical profiles.

Secondary activity contributes to the distinctive phencyclidine clinical syndrome:

• Sigma-1 receptor agonism, shared in part with dextromethorphan and methoxetamine. The sigma-1 receptor is a chaperone protein at the endoplasmic reticulum-mitochondria interface; its role in psychotomimetic effects is incompletely characterized but plausibly contributes to the prolonged psychosis that phencyclidine produces and ketamine generally does not.^{[citation needed]}
• Dopamine transporter inhibition, weak. Contributes modestly to the adrenergic and reinforcing dimensions of the experience.
• Nicotinic acetylcholine receptor antagonism. May contribute to autonomic disturbance.

The two pharmacologic features that most distinguish phencyclidine from ketamine clinically are (a) substantially longer NMDA-receptor unblock kinetics, producing a longer duration of action, and (b) the sigma-1 component, contributing to a more sustained and less medically tolerable post-acute psychiatric tail.

Pharmacokinetics

Phencyclidine is a highly lipophilic weak base (pKa ~8.5). It is rapidly absorbed by all routes, with smoked bioavailability ~85% and oral bioavailability ~72%. Distribution is wide, with substantial deposition into adipose tissue and into the brain (brain concentrations several-fold higher than plasma). The volume of distribution is correspondingly large (~6 L/kg).

The elimination half-life is highly variable (reported range 7-46 hours, mean ~21 h) and prolonged compared with ketamine. The lipophilic deposition behaves as a slow-release reservoir, and re-release from fat is one mechanism for the prolonged and sometimes biphasic clinical course that phencyclidine produces. Metabolism is hepatic, predominantly by CYP3A4 and CYP2B6, producing inactive hydroxylated metabolites that are renally excreted. The parent compound is the principal active species; urinary acidification accelerates renal elimination of the parent (because more of the weak-base parent is ion-trapped in acidic urine), although this is no longer recommended in management because of the Rhabdomyolysis risk that acidic urine compounds.

Acute effects and clinical presentation

Phencyclidine intoxication produces a syndrome that is recognizable once seen but that mimics, at different points along its dose-response curve, several other emergency-medicine presentations. The full syndrome includes:

• Dissociation: a sense of being separated from one's body, from the environment, or from the experience of meaning. At low doses, dreamy and disinhibited; at higher doses, the dissociation becomes complete and the user is unresponsive to environmental cues.
• Anesthesia and analgesia: profound. Self-injury without subjective pain is the toxicologic basis of the "superhuman strength" mythology.
• Vertical nystagmus: the most clinically distinctive sign. Phencyclidine produces nystagmus in all three planes (vertical, horizontal, rotary), but vertical nystagmus in particular narrows the differential strongly toward dissociative intoxication.
• Adrenergic surge: tachycardia, hypertension, mydriasis, diaphoresis.
• Motor disturbance: ataxia at low doses; muscle rigidity, catatonia, and dystonic posturing at higher doses; opisthotonic posturing and seizures at toxic doses.
• Hyperthermia: frequently overlooked, sometimes fatal; the agitated, hypermetabolic, often-restrained patient can reach core temperatures of 41-42 C.
• Hyperreflexia and hyperacusis: useful in distinguishing phencyclidine from anticholinergic delirium (in which reflexes are normal or depressed) and from sympathomimetic intoxication (in which dissociation is absent).

The clinical course is typically 4 to 8 hours for acute behavioral effects, with a recovery period of 24 to 48 hours during which cognitive, perceptual, and affective abnormalities can wax and wane unpredictably as phencyclidine re-releases from adipose stores.

Toxicology and emergency management

The acute toxicology of phencyclidine intoxication centers on five complications, each potentially fatal:

1. Hypertensive emergency with end-organ damage (intracranial hemorrhage, aortic dissection).
2. Hyperthermia progressing to rhabdomyolysis and acute kidney injury.
3. Rhabdomyolysis from sustained muscular hyperactivity, often compounded by physical restraint and by acidic environments (urine acidification, lactic acidosis from prolonged seizure or exertion).
4. Seizures, from focal twitching at intermediate doses to status epilepticus at toxic doses.
5. Traumatic injury sustained while dissociated and analgesic, frequently undetected until imaging in the ED.

Management is supportive and follows several principles:

• Quiet, dim, low-stimulation environment wherever feasible. External stimulation worsens agitation and worsens hyperthermia.
• Benzodiazepines as first-line for agitation, seizure, hypertension, and hyperthermia. Lorazepam or diazepam IV, titrated to effect; very high cumulative doses may be required.
• Neuroleptics are generally avoided in acute phencyclidine intoxication. They lower seizure threshold, worsen hyperthermia by impairing thermoregulation, do not address the underlying NMDA-mediated state, and have not demonstrated benefit over benzodiazepines.
• Aggressive IV hydration to maintain urine output and protect renal function in the setting of rhabdomyolysis.
• Active cooling for hyperthermia.
• Urinary acidification is no longer recommended. Once routine, it was abandoned because the acceleration of renal phencyclidine elimination is small relative to the worsening of rhabdomyolysis-induced renal injury.
• Differential diagnosis must include sympathomimetic intoxication (Methamphetamine, Cocaine, synthetic cathinones), anticholinergic delirium, Serotonin syndrome, Neuroleptic malignant syndrome, and acute primary psychosis. Vertical nystagmus, dissociation, and hyperreflexia together favor phencyclidine.

Psychiatric effects and the NMDA model of schizophrenia

Phencyclidine reliably produces psychosis. Three patterns are distinguished clinically:

• Acute phencyclidine psychosis: during and shortly after acute intoxication, lasting hours. Hallucinations, delusions, severe disturbance of self-experience. Resolves with the acute pharmacologic effect.
• Prolonged phencyclidine psychosis: persistent psychosis for days to several weeks following a single intoxication episode. Sometimes responsive to neuroleptics; often runs its course regardless. Whether it represents a pharmacologic tail (lipophilic re-release) or an unmasking of latent vulnerability is debated.
• Chronic phencyclidine-associated schizophreniform disorder: among heavy chronic users, a syndrome closely indistinguishable from chronic schizophrenia that may persist even after sustained abstinence.

This clinical phenomenology is the empirical basis of the NMDA hypofunction model of schizophrenia, the alternative to the dopamine model that has dominated schizophrenia neuroscience since the 1990s. The NMDA model accounts for features the dopamine model cannot: the negative symptoms (affective flattening, alogia, social withdrawal), the cognitive deficits (working memory, executive function), and the resistance of these features to D2-antagonist treatment. It also generates testable therapeutic predictions, several of which (glycine-site agonists, mGluR2/3 agonists, glycine transporter inhibitors) have been pursued through clinical development with mixed results.

The same receptor pharmacology underlies Ketamine's breakthrough as a rapid-acting antidepressant. The 2022 FDA approval of dextromethorphan-bupropion (Auvelity), an NMDA antagonist combined with a CYP2D6 inhibitor to extend its half-life, is a direct clinical descendant of the line of research that began when Greifenstein and Domino first noticed that something strange was happening to Sernyl patients in the recovery room.

Recreational use and street forms

Phencyclidine has moved through several distinct recreational eras, each shaped by formulation and route:

• Tablet era (1967-early 1970s): the original "PeaCePill" in San Francisco. Oral phencyclidine produces a slower, more dose-uncertain experience that users generally rated as unpleasant. The tablet form did not establish a sustained market.
• Dipped-cigarette era (mid-1970s to 1980s): liquid phencyclidine, often dissolved in ether, formaldehyde, or other solvents, applied to a cigarette or cannabis joint and smoked. Names: wet, fry, sherm (after Sherman brand cigarettes used as the substrate), dipper. Pharmacokinetically more controllable than oral dosing; the smoked route allows the user to titrate dose by puff. This was the formulation that drove the urban epidemic and the rescheduling.
• Powder era (overlapping): angel dust as a white-to-off-white powder, sometimes snorted, sometimes added to cannabis, sometimes sold as something else (mescaline, THC, or LSD analogs, all of which it is not pharmacologically resemble).
• Embalming-fluid confusion: some street samples genuinely contained formaldehyde as a solvent for phencyclidine; other samples sold as "embalming fluid" or "wet" were formaldehyde or PCP-free dipping solutions with no phencyclidine content. Users could not reliably know which they were getting; clinicians presented with intoxicated patients could not reliably know either.

Current US recreational phencyclidine use is at low ebb relative to its 1980s peak. The dissociative scene has shifted toward Ketamine (medical diversion and clandestine synthesis), toward the structural analogs 3-MeO-PCP and 4-MeO-PCP in the research-chemical market, and historically toward Methoxetamine (MXE), which was widely available 2010-2013 before being scheduled in most jurisdictions.

Analogs and the modern dissociative scene

The arylcyclohexylamine scaffold is generative. Several phencyclidine analogs have appeared as research chemicals since 2008, including:

• 3-MeO-PCP (3-methoxyphencyclidine): longer-acting and reportedly more selectively psychotomimetic than the parent. Scheduled in many jurisdictions.
• 4-MeO-PCP (4-methoxyphencyclidine): less potent and shorter-acting; sometimes available where 3-MeO-PCP is scheduled.
• PCEEA, PCMEA, PCMPA: minor variations on the piperidine substituent.
• Methoxetamine (MXE): a ketamine analog with longer duration and a more pronounced psychotomimetic profile, sometimes described as bridging the ketamine-phencyclidine gap. scheduled in the UK in 2012 under a temporary class order; scheduled internationally.
• Deschloroketamine, 2-FDCK, and other ketamine analogs occupy adjacent space in the modern dissociative market.

The pharmacology of these analogs is incompletely characterized and routinely mischaracterized in the marketplace; pre-purchase claims about potency, duration, and toxicity often diverge from what users report in case reports and forum experience.

Scientific and clinical legacy

Phencyclidine is the rare withdrawn medicine whose post-withdrawal scientific value substantially exceeded its therapeutic value. The lines of work it opened:

• Identification of the NMDA receptor as a discrete pharmacologic target, and discovery of the PCP site as a probe of receptor activation state.
• The glutamate hypothesis and the NMDA hypofunction model.
• The Ketamine antidepressant program, beginning in 2000 with Robert Berman and colleagues at Yale and culminating in the 2019 approval of esketamine (Spravato) for treatment-resistant depression.
• The dextromethorphan-bupropion approval (2022) for major depressive disorder, extending NMDA-targeted treatment beyond ketamine.
• A generation of glutamatergic schizophrenia therapeutic candidates (rapastinel, bitopertin, pomaglumetad), most of which failed in late-phase development, but which would not have been pursued at all without the phencyclidine-derived model.

See also

• Ketamine - the structural analog that succeeded clinically where phencyclidine failed
• Methoxetamine - a research-chemical-era analog
• Dextromethorphan - clinically used NMDA antagonist with shared mechanism
• Memantine - NMDA antagonist with low-affinity, fast-off kinetics permitting therapeutic use in dementia
• Auvelity - dextromethorphan-bupropion, clinical descendant of the NMDA antidepressant program
• Esketamine (Spravato) - the FDA-approved enantiomer of ketamine for treatment-resistant depression
• NMDA receptor - the molecular target