Cimetidine

An H2 blocker acid reducer, FDA-approved for duodenal and gastric ulcers, erosive GERD and acid hypersecretion (Zollinger-Ellison); lower doses sold over the counter for heartburn.

psilodex.org/m/cimetidine · data v2026.10.05

Clinical contraindication: None listed; No label contraindication covering psilocybin was found. This is not a finding that the combination has been shown to be without risk.
Psilocybin evidence: No psilocybin data
Half-life: ~2 h; Approximate, in adults. Source under Interaction below.

Named in state rules?

Not namedColorado

Not namedOregon

PendingNew Mexico

Cite asPsilodex, “Cimetidine”, v2026.10.05. psilodex.org/m/cimetidine

Clinical data and state rules are separate facts. Both are shown; neither is a decision.

What it is used for

FDA status

FDA-approved

Prescription cimetidine tablets are FDA-approved; lower-dose cimetidine (e.g. Tagamet HB) is sold over the counter for heartburn.

FDA-approved for

Duodenal ulcer (treatment and maintenance)
Benign gastric ulcer
Erosive gastroesophageal reflux disease (GERD)
Pathological acid hypersecretion (e.g. Zollinger-Ellison syndrome)

Sources [1]

01 Interaction with psilocybin

It may slightly slow how the body clears psilocybin. Separately, it has rarely caused confusion or hallucinations, mainly in very ill people.

Detail Cimetidine slows the liver's breakdown of many drugs by inhibiting microsomal (CYP) enzymes. CYP2D6 and CYP3A4 metabolized psilocin in lab studies (Thomann 2024). So cimetidine may raise psilocin levels, though the body removes psilocin mainly by glucuronidation (a liver process) and conversion to 4-HIAA. There are no psilocybin data. Separately, its label reports reversible confusional states, including agitation, psychosis and hallucinations, mainly in severely ill or older people. Sources [1][2]

Human data with psilocybin: No psilocybin data; based on how it works and its label
Half-life: ~2 h (FDA label, checked 2026-10-04)

Evidence types on this record

No psilocybin dataNo psilocybin data: no human data with psilocybin or another classic psychedelic; the text draws on the medication's own pharmacology and its label or reference facts

02 Pharmacology relevant to psilocybin

1 listed · counted on reports

Psychiatric reactions: The label or source warns of psychiatric effects such as psychosis, mania, hallucinations, suicidal thoughts or severe mood or behavior changes. Psilocybin can also bring up strong emotions, so these warnings are relevant to screening. Sources [1]

03 State rules

Plain-language summary · official text governs

CO4 CCR 755-1

Not named

No provision names this medication or its class.

Colorado · checked 2026-10-04

OROAR 333-333

Not named

No provision names this medication or its class.

Oregon · checked 2026-10-04

NM7.35.3 NMAC (proposed)

Pending

New Mexico's clinical rules (proposed 7.35.3 NMAC) are not final. No provision naming this medication has been found.

New Mexico · checked 2026-10-04

04 Conditions to ask about

None linked

No condition named in state rules is linked to this medication.

05 Sources

Cimetidine tablets (Teva) · FDA label (DailyMed) — revised 2026-09 | Psychiatric reactions: Precautions, General "Reversible confusional states (see ADVERSE REACTIONS) have been observed on occasion, predominantly, but not exclusively, in severely ill patients." Adverse Reactions, CNS "Reversible confusional states, e.g., mental confusion, agitation, psychosis, depression, anxiety, hallucinations, disorientation, have been reported predominantly, but not exclusively, in severely ill patients." | Precautions, Drug Interactions "Cimetidine, apparently through an effect on certain microsomal enzyme systems, has been reported to reduce the hepatic metabolism of warfarin-type anticoagulants" | "The half-life of cimetidine is approximately 2 hours."
Thomann J, Kolaczynska KE, Stoeckmann OV, et al. In vitro and in vivo metabolism of psilocybin’s active metabolite psilocin. Front Pharmacol 2024;15:1391689. — Abstract: "recombinant CYP2D6 and CYP3A4 enzymes metabolized nearly 100% and 40% of psilocin, respectively." "MAO-A transformed psilocin into minimal amounts of 4-HIAA and 4-HTP." "In conclusion, MAO-A, CYP2D6, and CYP3A4 are involved in psilocin's metabolism." Introduction: "Around 33% of a psilocybin dose is renally excreted as 4-HIAA."

Record history

v2026.10.02First published.
2026-10-04Last reviewed against its sources.

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