An essential amino acid and the dietary precursor to serotonin and melatonin. Found in all complete proteins, and sold in free form for mood and sleep.
Tryptophan carries the most serious contamination history in the supplement industry. In 1989 an epidemic of eosinophilia-myalgia syndrome — over 1,500 recorded cases and at least 37 deaths in the US — was traced to L-tryptophan supplements, and epidemiological investigation linked cases to product from a single Japanese manufacturer that had changed its bacterial fermentation strain and reduced a purification step, generating trace contaminants including a compound designated peak E. The FDA banned import and restricted sale for over a decade. Manufacturing has changed and the product returned to shelves, but the episode remains the clearest demonstration that a supplement can be lethal because of how it was made rather than what it is.
Sold as
Capsule, Tablet, Powder
Standardised to
mg L-tryptophan; pharmaceutical-grade purity is the material question — a product without this marker may not resemble what was studied.
Also known as
Tryptophan, L-tryptophan
Regulation: Following the 1989 eosinophilia-myalgia outbreak the FDA restricted import and sale of L-tryptophan supplements; restrictions were relaxed in 2001 and 2005 and it is again sold as a dietary supplement without pre-market efficacy or safety review. It remains a prescription medicine in some countries, used as an adjunct in depression under supervision.
What the research used
The amounts, forms and durations published trials of L-tryptophan actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Sleep onset studies
1–5 g
Limited
Single evening dose
Older studies report reduced sleep onset latency at 1 g and above. The literature is largely from the 1970s and 1980s and predates modern trial standards.
Studied for Mood trials
2–6 g
Not enough
Divided doses daily
Recorded as tested rather than established. Systematic review of tryptophan and 5-HTP for depression found the evidence insufficient.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🌙Evening
Limited
Taken in the evening for sleep, and away from a protein meal because tryptophan competes with other large neutral amino acids for blood-brain barrier transport — a small carbohydrate load raises the ratio in tryptophan’s favour. The transport competition is established pharmacology and has been measured directly.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Limited
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Older controlled studies report reduced time to fall asleep with 1 g or more of tryptophan, particularly in people with mild insomnia. The literature is dated, the trials are small, and it has not been re-examined to modern standards.
What may be going on: Tryptophan is hydroxylated to 5-HTP and decarboxylated to serotonin, which is converted to melatonin in the pineal gland. Its entry to the brain depends on its ratio to other large neutral amino acids.
Limited evidence. Early human work, or mostly lab and animal research.
Systematic review of tryptophan and 5-HTP for depression concluded that the evidence was insufficient, with almost all trials failing quality criteria. Acute tryptophan depletion lowers mood in vulnerable people, which is a different experiment and does not establish that supplementation raises it.
Not enough evidence. Not enough human research to say anything useful yet.
Meta-analysis · The Australian and New Zealand journal of psychiatry · 2002 · PMID 12169147
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Worth knowing
The 1989 eosinophilia-myalgia syndrome outbreak
More than 1,500 cases and at least 37 deaths in the US in 1989 were traced to L-tryptophan supplements, with cases concentrated in product from one manufacturer that had altered its fermentation strain and reduced a purification step. Contaminants including a compound designated peak E were identified in implicated lots. It is the most consequential supplement contamination event on record, and the reason manufacturing quality is not a peripheral concern.
Interaction
Serotonin syndrome
Interacts withSSRIs, SNRIs, MAO inhibitors, tricyclic antidepressants, triptans, tramadol, lithium
Tryptophan combined with SSRIs, SNRIs, MAO inhibitors, tricyclics, triptans, tramadol or lithium can precipitate serotonin syndrome. The risk is the same class of problem as with 5-HTP.
Sedative effects may add to benzodiazepines, sedating antihistamines, alcohol and other central nervous system depressants.
Worth knowing
Carcinoid and liver disease
Tryptophan metabolism is altered in carcinoid syndrome and in liver disease, and supplementation is inappropriate in both without medical supervision.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.