The short answer
Do not use an L-tryptophan supplement with isocarboxazid unless the prescribing clinician has explicitly planned and monitored it. The current Marplan label says tryptophan may precipitate hypertension and related symptoms. Reports with other MAO inhibitors also describe neurologic toxicity and hypomania.
What this answer covers
Oral L-tryptophan supplements with oral isocarboxazid. This does not cover normal dietary protein or 5-hydroxytryptophan, which is a distinct substance.
- Supplement or preparation
- L-Tryptophan
- Medication ingredient
- isocarboxazid
The label names tryptophan
Marplan prescribing information lists tryptophan among compounds that may precipitate hypertension, headache and related symptoms. This is an exact-drug warning for oral isocarboxazid.
Other reported reactions are different
A small phenelzine case series describes myoclonus, hyperreflexia and sweating after L-tryptophan was added. A separate two-case abstract reports hypomania with an MAO inhibitor but does not identify each person’s exact drug. Hypomania is a mood state, not another name for serotonin syndrome.
The studies do not prove an exact event rate
The clinical reports involved other MAO inhibitors and only a few patients. They support the reason for caution but cannot predict how often a reaction occurs with isocarboxazid.
Do not rely on dose spacing
A few hours between doses has not been shown to prevent the interaction. Discuss any supplement plan with the isocarboxazid prescriber. Seek urgent care for a sudden severe headache, chest pain, marked palpitations, confusion, severe agitation, muscle jerking or marked stiffness.
Evidence and practical considerations
Side effects & toxicity
Current Marplan labeling names tryptophan among compounds that may precipitate hypertension, headache and related symptoms.
- Exact scope
- Oral L-tryptophan supplements, not 5-hydroxytryptophan. with Oral isocarboxazid tablets.
- Medication form and route
- oral
What remains uncertain
- No isocarboxazid-specific incidence or controlled exact-pair trial was identified.
Factors that may matter: Dose and co-medications were not quantified in the label..
Read the supporting source summaries
Source 1: Current U.S. prescribing information
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Patients prescribed oral Marplan tablets for depression.
The label says excessive caffeine should be avoided. It separately states that tyrosine and tryptophan with Marplan may precipitate hypertension, headache, and related symptoms, and describes a neurologic and behavioral syndrome reported with MAO inhibitors plus tryptophan. It also warns that serious reactions may occur when MAO inhibitors are given with serotonergic drugs, with examples that do not include Hypericum perforatum.
How this applies: Primary authority for current oral isocarboxazid precautions. Applicability is direct only where the exact supplement is named.
The label gives no safe caffeine threshold and no exact-pair event rates. It does not name Hypericum perforatum. Its tryptophan syndrome summary includes MAO-inhibitor class reports rather than an identified isocarboxazid case series.
Research assessment: · Open full citations ↗
Side effects & toxicity
MAO inhibitor plus L-tryptophan reports describe neurologic toxicity and hypomania. Neither report directly establishes serotonin syndrome with isocarboxazid.
- Exact scope
- Oral L-tryptophan. with The neurologic case series used phenelzine. The hypomania abstract says MAO inhibitor without assigning its indexed phenelzine and tranylcypromine terms to individual cases. Applicability to isocarboxazid is class-based.
- Medication form and route
- oral
What remains uncertain
- Small case reports with other or unspecified individual MAO-inhibitor regimens and abstract-only access.
Factors that may matter: Individual psychiatric history may affect interpretation of hypomanic symptoms..
Read the supporting source summaries
Source 1: Current U.S. prescribing information
dailymed.nlm.nih.gov · Source check Sep 10, 2026
Population: Patients prescribed oral Marplan tablets for depression.
The label says excessive caffeine should be avoided. It separately states that tyrosine and tryptophan with Marplan may precipitate hypertension, headache, and related symptoms, and describes a neurologic and behavioral syndrome reported with MAO inhibitors plus tryptophan. It also warns that serious reactions may occur when MAO inhibitors are given with serotonergic drugs, with examples that do not include Hypericum perforatum.
How this applies: Primary authority for current oral isocarboxazid precautions. Applicability is direct only where the exact supplement is named.
The label gives no safe caffeine threshold and no exact-pair event rates. It does not name Hypericum perforatum. Its tryptophan syndrome summary includes MAO-inhibitor class reports rather than an identified isocarboxazid case series.
Source 2: Human case reports
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Two people receiving an unspecified MAO inhibitor in the accessible abstract; neither case is identified there as isocarboxazid exposure.
Two cases of hypomania were reported after L-tryptophan was added.
How this applies: Supports a general MAOI plus L-tryptophan mood-effect concern while preserving the diagnostic and identity limits.
Abstract only; two cases; individual MAO-inhibitor regimens are not identified in the abstract. Hypomania is not the same diagnosis as serotonin syndrome or hypertension.
Source 3: Human case series
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Three patients on an MAO inhibitor; the article title identifies phenelzine-tryptophan combination treatment, not isocarboxazid.
Transient myoclonus, hyperreflexia, jaw quivering, teeth chattering and diaphoresis were reported.
How this applies: Indirect clinical corroboration for neurologic toxicity from L-tryptophan with an MAO inhibitor.
Abstract only; three cases; no isocarboxazid exposure; does not quantify hypertension.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Initial targeted source-checked draft. Current claims were adjudicated against the current Marplan label and inspected clinical literature. Clinical review has not occurred. | |
| Research summary published |
Article revision: d70b50ad5a8329c0
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.