Hallucinogens — LSD, PCP, Ketamine

USMLE Step 1 trap: Conflates LSD and PCP intoxication, missing PCP's nystagmus, analgesia, and violent dissociation. LSD causes visual hallucinations with intact orientation and no analgesia, while PCP causes dissociation, nystagmus (vertical and horizontal), analgesia, and violent behavior due to NMDA receptor antagonism.

Hallucinogens show up on USMLE Step 1 primarily as a clinical differentiation problem — the exam loves presenting a patient with altered behavior, hallucinations, or agitation and asking you to identify the substance or select the correct management. LSD, PCP, and ketamine are grouped together conceptually because all three distort perception, but their mechanisms, presentations, and treatments are meaningfully different. The exam tests this at the application level: you'll get a vignette describing a patient brought in by police, acting violently, with abnormal eye movements and apparent indifference to pain, and you need to recognize that this is PCP — not LSD, not amphetamines. The distinguishing features are specific and learnable.

What makes this tricky is that students lump these drugs together under 'hallucinogens' and assume similar presentations and management. That shortcut will cost you points. PCP (phencyclidine) is an NMDA receptor antagonist — that mechanism drives its unique features: dissociation, nystagmus (especially vertical, which is nearly pathognomonic), analgesia, and paradoxical violent behavior. LSD works through 5-HT2A agonism and causes vivid visual hallucinations with a calm, oriented patient who knows something strange is happening. The exam also tests management: students reflexively reach for antipsychotics for any agitated psychiatric presentation, which is the wrong move for PCP.

Ketamine is the third piece here, and USMLE Step 1 increasingly tests its modern psychiatric use. As an NMDA receptor antagonist structurally related to PCP, it's been repurposed as intranasal esketamine (Spravato) for treatment-resistant depression — with effects appearing within hours rather than weeks. If a question mentions rapid antidepressant onset or treatment-resistant depression, ketamine should be on your radar. Understanding the shared mechanism across PCP and ketamine (NMDA antagonism) while keeping their clinical contexts separate is the key conceptual move here.

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Common misconceptions

Common mistake

Wrong: LSD and PCP intoxication present identically because both cause hallucinations.

Right: LSD causes visual hallucinations with intact orientation and no analgesia, while PCP causes dissociation, nystagmus (vertical and horizontal), analgesia, and violent behavior due to NMDA receptor antagonism.

Both drugs cause perceptual disturbances, but the underlying mechanisms are completely different and produce distinct clinical pictures. LSD is a 5-HT2A agonist — patients hallucinate vividly (usually visual), but they remain oriented, can often recognize that the experience is drug-induced, and do not develop analgesia or nystagmus. PCP is an NMDA receptor antagonist acting at a dissociative level — it blocks glutamate signaling, producing true dissociation from reality, vertical nystagmus, marked analgesia (patients can sustain serious injuries without reacting), and a tendency toward violent, unpredictable behavior. When you see nystagmus plus analgesia plus violence, that triad points to PCP every time.

Common mistake

Wrong: Antipsychotics are the first-line treatment for PCP-induced agitation.

Right: Benzodiazepines are preferred for PCP agitation; antipsychotics (especially low-potency agents) can lower the seizure threshold and worsen anticholinergic effects in PCP intoxication.

The instinct to give antipsychotics for agitation makes sense in other contexts, but PCP intoxication is a specific exception. PCP already lowers the seizure threshold and has some anticholinergic activity; adding a low-potency antipsychotic (like chlorpromazine) piles on anticholinergic effects and further increases seizure risk. Benzodiazepines are the correct first-line choice — they control agitation, reduce seizure risk, and avoid the dangerous interaction. If antipsychotics appear on an answer list for PCP agitation, treat them as a trap, especially the low-potency phenothiazines.

Common mistake

Gap: Unaware that ketamine/esketamine has a current psychiatric indication for treatment-resistant depression

Ketamine (an NMDA receptor antagonist) has FDA approval for treatment-resistant depression as intranasal esketamine (Spravato), with rapid antidepressant effects within hours.

Ketamine has moved beyond anesthesia and is now a legitimate psychiatric treatment. Esketamine (the S-enantiomer of ketamine), administered intranasally as Spravato, received FDA approval for treatment-resistant depression — defined as failure of at least two adequate antidepressant trials. The key distinguishing feature is speed: traditional antidepressants take 4-6 weeks; esketamine can produce measurable antidepressant effects within hours to days. The mechanism is NMDA receptor antagonism, which is thought to rapidly restore synaptic plasticity. If a Step 1 question mentions treatment-resistant depression or rapid antidepressant onset, esketamine/ketamine is the answer they're pointing toward.

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What the exam tests

Given a vignette describing a patient with violent behavior, nystagmus, and analgesia versus one with visual hallucinations and preserved orientation, identify which features belong to PCP versus LSD intoxication and explain the mechanism driving each.
Select the correct first-line medication for PCP-induced agitation and identify why low-potency antipsychotics are specifically contraindicated in this setting.
Recognize ketamine's mechanism of action as an NMDA receptor antagonist and identify its current FDA-approved psychiatric indication (intranasal esketamine for treatment-resistant depression), including its characteristic rapid onset of action.

Can you avoid these mistakes?

A 22-year-old man is brought to the ED by police after being found fighting on the street. He appears confused, does not respond to a painful sternal rub, and has eye movements that go in both horizontal and vertical directions. What substance is most likely responsible, and what receptor mechanism explains the analgesia?

You are asked to manage the agitation of a patient with confirmed PCP intoxication. A colleague suggests starting haloperidol, while you consider lorazepam. Who is correct, and what specifically makes low-potency antipsychotics dangerous in this context?

A 45-year-old woman with major depressive disorder has failed three adequate antidepressant trials. Her psychiatrist discusses a recently FDA-approved treatment that works within hours. What drug class is this, what is its mechanism, and how does it differ mechanistically from SSRIs?

Contrast the expected behavior of a patient intoxicated on LSD versus one intoxicated on PCP: which is more likely to be oriented, which is more likely to be violent, and what physical exam finding would strongly favor PCP over all other options?

Hallucinogens — LSD, PCP, Ketamine

Common misconceptions

What the exam tests

Can you avoid these mistakes?

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