5-HTP and antidepressants: why they don't mix

Of all the notes on 5-HTP, one is not negotiable: not together with antidepressants. That is not a formula set down as a precaution but a contraindication in the product monograph of the Canadian natural health products directorate, and it reads the same way at the German consumer advice centre — 5-HTP must not be taken together with other products that act on serotonin levels. The reason lies in the metabolism: 5-HTP supplies raw material for exactly the messenger substance whose availability these medicines work on.

Besides the antidepressants there is a second, less familiar group — medicines that slow the breakdown of 5-HTP in the body, carbidopa above all — and a third point that many people overlook: the vitamin B6 that sits in almost every 5-HTP preparation brings an interaction of its own.

Why 5-HTP interacts at all

The path is short. The amino acid L-tryptophan becomes 5-HTP, 5-HTP becomes serotonin, and from serotonin the body can form melatonin. The decisive difference between the two precursors: L-tryptophan still has to pass the slowest step of that chain, while 5-HTP already has it behind it. A single enzyme — aromatic L-amino acid decarboxylase — converts it straight into serotonin, and not only in the brain but to a considerable extent before that, in the gut and the liver.

That makes 5-HTP not a nutrient that is stored somewhere but raw material for a messenger substance. Everything else that reaches into the same household — providing more serotonin, inhibiting its reuptake or blocking its breakdown — adds to it. How quickly that happens is set out in 5-HTP: what to know about taking it: peak blood levels are reached after one to two hours.

The medicines involved

The Canadian monograph (as of 28 August 2024) distinguishes two levels: a true contraindication and a duty to consult. Sorted, it looks like this:

Agent	Examples	What the warning requires
Antidepressants	SSRIs, SNRIs, tricyclics, MAO inhibitors	Contraindication — do not use
Carbidopa	component of Parkinson's medicines	consult a physician before use
Cough medicines with dextromethorphan	over-the-counter cough suppressants	consult a physician
Painkillers with serotonergic action	e.g. tramadol	consult a physician
Anti-nausea medicines	setrons, metoclopramide	consult a physician
Acute migraine medicines	triptans	consult a physician
Herbal mood preparations	St John's wort	consult a physician

Two things stand out. First, half the list sits on the shelf without a prescription: a cough syrup with dextromethorphan gets bought without anyone thinking about serotonin. Second, the list is not graded by milligrams. The warnings apply to 5-HTP preparations regardless of whether 12.5 mg or 200 mg sit in the daily dose — what separates those amounts, and how they can be compared at all, is set out in 5-HTP dosage: how much is actually in there?.

On the question of how much time has to pass after stopping an antidepressant, none of the official sources says anything. That is not a gap to be filled with a rule of thumb from the internet: active substances differ in this by orders of magnitude, and the decision belongs with the doctor who prescribed the medicine.

Carbidopa: the special case with a history of its own

Carbidopa is on the list for a different reason. The substance inhibits exactly the decarboxylase that converts 5-HTP — but only outside the brain. In Parkinson's medicines that is intended, because it means more levodopa arrives where it is supposed to. For 5-HTP the same inhibition means: it is barely broken down in the periphery, and a considerably larger share reaches the central nervous system than it otherwise would.

This combination has been studied medically, and it is where the most striking warning of all comes from. In 1980 a case report in the New England Journal of Medicine described a scleroderma-like illness in a patient who was given 5-HTP together with carbidopa. Two points of today's monograph go back to that: the contraindication "do not use in scleroderma" and the instruction to stop taking the product and seek medical advice if hardening of the skin, pain or stiffness in muscles and joints, tremor, confusion or an accelerated heartbeat occur. For a food supplement that is an unusually concrete warning — it describes what to watch for, not merely that one should be careful.

What the vitamin B6 in the same product means

That our 5-HTP + Vitamin B6 and many comparable preparations contain vitamin B6 has a factual reason: pyridoxal-5-phosphate, the active form of the vitamin, is the cofactor of that very decarboxylase which converts 5-HTP. Without B6 the enzyme does not work.

Precisely from this, though, comes an interaction of its own that has nothing to do with serotonin: vitamin B6 and levodopa. If levodopa is taken without a decarboxylase inhibitor, pyridoxine speeds up its breakdown while it is still in the blood, and less of it arrives in the brain. In the combination therapy with carbidopa or benserazide that is customary today this effect is largely defused — the question nevertheless remains one for the treating practice, not for weighing things up on your own in front of the shelf.

Per tablet (recommended daily dose)	Amount	% NRV*
5-HTP (from Griffonia seed extract)	12.5 mg	no reference value set
Vitamin B6	1.6 mg	114 %

* Nutrient reference values in accordance with Regulation (EU) No 1169/2011, Annex XIII.

For vitamin B6 the authorised statements read, in their official wording: Vitamin B6 contributes to normal functioning of the nervous system, Vitamin B6 contributes to normal psychological function and Vitamin B6 contributes to the reduction of tiredness and fatigue (Regulation (EU) No 432/2012). For 5-HTP itself there is no authorised statement: EFSA assessed the claims submitted for it and did not consider them substantiated. Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle.

When several preparations stand side by side

The second calculation that goes wrong in everyday life concerns not medicines but your own shelf. Anyone taking several food supplements adds up ingredients without noticing:

• Vitamin B6 appears twice. A B-complex or a multivitamin brings its own amount along. In 2023 EFSA set the tolerable upper intake level at 12 mg per day for adults (EFSA Journal 2023;21(5):8006); one tablet of our preparation contributes 1.6 mg to that.
• St John's wort counts among the serotonergic agents and stands in the monograph expressly on the same line as the medicines — even though it is sold freely as a tea or a capsule.
• Melatonin is chemically related, but not a documented interaction partner. It is formed in the body from serotonin, so it stands at the end of the same chain. There is no documented interaction with 5-HTP; what applies to both is the note on possible drowsiness. Anyone who has both in the cupboard sticks to the respective recommended intake and does not add together what is written on two packages — an overview of the strengths is in the melatonin category.
• Combination products count twice. A product such as Ashwagandha with Melatonin contains two substances in one tablet. For the question of what adds up overall, the ingredient list is what counts, not the number of doses.

Not on this list are magnesium, valerian or ashwagandha: under the warnings cited here they do not count as serotonergic agents. That does not mean anything may be combined at will — it means that the warning at issue here does not concern them.

Drowsiness, alcohol and driving

The monograph carries one further note that applies in everyday life more often than all the medicine questions put together: while using it, caution is required at the wheel and around machinery, because drowsiness may occur. On alcohol there is no study of its own for 5-HTP; that two tiring influences add up is plausible but not the same as evidence — and we do not write down more here than is documented. Why taking it with a meal rather than on an empty stomach is nevertheless the usual recommendation is set out in 5-HTP: morning or evening, with or without food?.

Pregnant and breastfeeding women should seek medical advice before use, and the preparations are intended exclusively for adults aged 18 and over. Which variants exist is shown in the 5-HTP category.

Common questions

Does the contraindication also apply to a preparation with only 12.5 mg? Yes. The monograph's warnings are not graded by amount, and the consumer advice centre likewise words its note without a quantity threshold. Less 5-HTP means less raw material for the same metabolic pathway — but not that the pathway is a different one.

Do I have to stop 5-HTP before an operation? The sources cited here say nothing about this. What makes sense in any case is to name the preparation in the pre-operative consultation, just like any other food supplement. Anaesthetics and painkillers partly belong to the serotonergic substances, and that assessment is made by the clinic, not by the package insert.

Is L-tryptophan the more harmless alternative? It is the precursor of 5-HTP, so it acts on the same pathway — only one step earlier and with a regulating step in between. Legally, different rules apply to the two in the EU. "Harmless" is therefore the wrong category; the question of other serotonergic agents arises with both.

I take an antidepressant and would still like to try something. Then 5-HTP is the wrong product, and there is no room for discretion in that. The contraindication is one of the few unambiguous statements in this field. Everything else belongs in the conversation with the practice that prescribed the antidepressant.

Sources: Health Canada / NNHPD, Monograph "5-HTP", as of 28.08.2024 · Verbraucherzentrale, "5-Hydroxytryptophan (5-HTP) aus der afrikanischen Schwarzbohne", as of 07.06.2024 · Sternberg EM et al., Development of a scleroderma-like illness during therapy with L-5-hydroxytryptophan and carbidopa, New England Journal of Medicine 1980;303(14):782 (PubMed 6997735) · Turner EH, Loftis JM, Blackwell AD, Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan, Pharmacology & Therapeutics 2006;109(3):325 (PubMed 16023217) · EFSA, Scientific Opinion … 5-hydroxytryptophan (5-HTP) and increase in satiety, EFSA Journal 2011;9(6):2198 · EFSA, Scientific Opinion … enhancement of mood (ID 1575) and attention (ID 1828), EFSA Journal 2009;7(10):1273 · EFSA, Tolerable upper intake level for vitamin B6, EFSA Journal 2023;21(5):8006 · Regulation (EU) No 432/2012 · Regulation (EU) No 1169/2011, Annex XIII.