Isoniazid

An antibiotic, FDA-approved for treating tuberculosis (with other drugs) and for preventing tuberculosis in people at higher risk.

psilodex.org/m/isoniazid · 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: Not stated in label or references

Named in state rules?

Not namedColorado

Not namedOregon

PendingNew Mexico

Cite asPsilodex, “Isoniazid”, v2026.10.05. psilodex.org/m/isoniazid

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

What it is used for

FDA status

FDA-approved

FDA-approved for

Tuberculosis (all forms, with other antituberculosis drugs)
Preventive therapy for tuberculosis infection in higher-risk groups

Sources [1]

Other safety

The label's boxed warning describes severe and sometimes fatal hepatitis, with risk rising with age and daily alcohol use.

About the medication itself, separate from any interaction with psilocybin.

Sources [1]

01 Interaction with psilocybin

Its label reports some MAO (an enzyme) inhibiting activity, so it may alter how the body clears psilocybin. Separately, it can cause severe hepatitis.

Detail There are no psilocybin data for isoniazid. Its label states it has some monoamine oxidase (MAO) inhibiting activity, seen as tyramine reactions with cheese or red wine. The label adds that it may inhibit diamine oxidase. Human data come from other MAOIs (MAO inhibitors). With LSD, long-term MAOI use was associated with decreased subjective response (Bonson & Murphy 1996). Psilocybin mushrooms taken during phenelzine produced no noticeable subjective effects (Barnett 2024, as summarized in Tap 2025). In lab studies, one group found MAO-A formed only small amounts of 4-HIAA from psilocin (Thomann 2024). Another group estimated that MAO-A accounts for about 81% of psilocin's Phase I liver metabolism (Chen 2025). About a third of a psilocybin dose is excreted as 4-HIAA (Thomann 2024), so MAO inhibition may raise psilocin exposure. Class data on blood pressure also exist. Tranylcypromine increased psilocybin's blood pressure and pupil effects in a small controlled study (Vojtěchovský 1968). One case report describes a hypertensive emergency (dangerously high blood pressure) with myocardial infarction (heart attack) (Barnett 2024). Separately, the label's boxed warning describes severe and sometimes fatal hepatitis. Sources [1][2][3][4][5][6][7]

Human data with psilocybin: No data on this medication itself; related drugs studied (see text)

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

2 listed · counted on reports

Liver injury: The label or source warns of liver injury, from raised liver enzymes to liver failure. Liver disease also affects how the body clears psilocin. Sources [1]
MAO inhibition: Inhibits monoamine oxidase, the enzyme that breaks down serotonin and other monoamines. MAO also helps clear psilocin, so it can change how strong and how long the psilocybin effect is, and it raises serotonin levels. 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

ISONIAZID tablet (Teva) · FDA label (DailyMed) — MAO inhibition: Precautions, Drug Interactions, Food "Because isoniazid has some monoamine oxidase inhibiting activity, an interaction with tyramine-containing foods (cheese, red wine) may occur." | Liver injury: Boxed Warning "Severe and sometimes fatal hepatitis associated with isoniazid therapy has been reported and may occur or may develop even after many months of treatment."
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."
Bonson KR, Murphy DL. Alterations in responses to LSD in humans associated with chronic administration of tricyclic antidepressants, monoamine oxidase inhibitors or lithium. Behav Brain Res 1996;73:229–233.
Barnett BS, Koons CJ, Van den Eynde V, et al. Hypertensive emergency secondary to combining psilocybin mushrooms, extended release dextroamphetamine-amphetamine, and tranylcypromine. J Psychoactive Drugs 2024;57(3):297–303.
Vojtěchovský M, Hort V, Šafratová V. Influence of MAO inhibitors on psilocybine induced psychosis. Activitas Nervosa Superior 1968;10(3):278–279 (via Tap 2025).
Tap SC, Thomas K, Páleníček T, et al. Concomitant use of antidepressants and classic psychedelics: a scoping review. J Psychopharmacol 2025. — Summary table, Barnett et al. (2024): phenelzine + nortriptyline dose "No (serious) AEs"; subjective effects at both doses "Complete absence of acute subjective effects." Bonson & Murphy row: "MAOIs were associated with decreases in subjective LSD response."
Chen J, Wang Z, Yong CY, et al. Elucidating the Phase I metabolism of psilocin in vitro. Arch Toxicol 2025;99(3):1085–1094. — Abstract: "MAO-A-mediated hepatic clearance of psilocin ... accounting for 80.9% of the total hepatic metabolism of psilocin" "MAO-A primarily contributed to the Phase I metabolism of psilocin."

Record history

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

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