The short answer
Avoid starting an L-tyrosine supplement while taking isocarboxazid unless the prescriber has specifically assessed it. The current Marplan label says tyrosine may precipitate hypertension and related symptoms. One monitored case with a different MAO inhibitor and parenteral nutrition had no reaction, so the risk is not quantified and may depend on the exposure.
What this answer covers
Oral L-tyrosine supplements with oral isocarboxazid. Food protein and tyrosine contained in parenteral nutrition are different exposures.
- Supplement or preparation
- L-Tyrosine
- Medication ingredient
- isocarboxazid
The current label names tyrosine
Marplan prescribing information lists tyrosine among compounds that may precipitate hypertension, headache and related symptoms. It does not provide a safe supplemental dose.
A contrary case used a different exposure
One patient receiving transdermal selegiline tolerated tyrosine contained in parenteral nutrition under monitoring. That differs from oral isocarboxazid plus a standalone L-tyrosine supplement and does not establish safety for this pair.
A retracted review is excluded
A 2014 review about L-tyrosine with phenelzine was retracted in 2021 because its central argument relied on retracted research. It is not used here as evidence that supplementation is safe or beneficial.
Spacing has not been validated
Taking L-tyrosine a few hours away from isocarboxazid has not been shown to remove the risk. Ask the prescriber before starting it. Seek urgent care for a sudden severe headache, chest pain, marked palpitations, neck stiffness, sweating, nausea or vomiting.
Evidence and practical considerations
Side effects & toxicity
Current Marplan labeling names tyrosine among compounds that may precipitate hypertension, headache and related symptoms.
- Exact scope
- Oral L-tyrosine supplements. with Oral isocarboxazid tablets.
- Medication form and route
- oral
What remains uncertain
- No controlled oral isocarboxazid plus L-tyrosine trial or event rate was identified.
Factors that may matter: Supplement dose and total tyrosine exposure may matter, but no threshold is established..
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 report
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: One 67-year-old woman receiving transdermal selegiline, not isocarboxazid.
The patient was monitored and tolerated the combination without hypertensive crisis.
How this applies: Contrary signal showing that risk is not inevitable across all tyrosine exposures and MAOI formulations.
One patient, parenteral nutrition rather than an oral L-tyrosine supplement, and transdermal selegiline rather than oral isocarboxazid. It cannot establish safety for this exact pair.
Source 3: Retraction notice
pmc.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Not a clinical population; publication-status notice for a 2014 phenelzine and L-tyrosine review.
The publisher states that the review’s central argument is no longer valid because it relied on retracted research.
How this applies: Requires exclusion of the 2014 review as positive evidence for safety or benefit.
A retraction notice supplies publication status, not positive or negative clinical evidence about isocarboxazid.
Research assessment: · Open full citations ↗
Physiologic interactions
One monitored patient tolerated tyrosine in parenteral nutrition with transdermal selegiline, a substantially different exposure that does not refute the Marplan warning.
- Exact scope
- Tyrosine within parenteral nutrition, not a standalone oral supplement. with Transdermal selegiline in the contrary report, not oral isocarboxazid.
- Medication form and route
- parenteral
What remains uncertain
- Single patient and clinically dissimilar exposure.
Factors that may matter: Route, MAO inhibitor selectivity and dose differ from the target pair..
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 report
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: One 67-year-old woman receiving transdermal selegiline, not isocarboxazid.
The patient was monitored and tolerated the combination without hypertensive crisis.
How this applies: Contrary signal showing that risk is not inevitable across all tyrosine exposures and MAOI formulations.
One patient, parenteral nutrition rather than an oral L-tyrosine supplement, and transdermal selegiline rather than oral isocarboxazid. It cannot establish safety for this exact pair.
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: ac67aa64574cd62a
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.