Amino Acid Supplement and Medicine Interactions: A Reference Table (Antidepressants, Nitrates, Blood Pressure Medicines, Thyroid Medicine, Levodopa)
A reference table of what the sources we read say about amino acid supplements and medicines, including antidepressants, nitrates, blood pressure and thyroid medicines and levodopa.
Content last updated . This page has not been medically reviewed.
Read this first
This page is information, not advice to start, stop or change anything. If you take a prescribed medicine, do not stop it on your own. MedlinePlus says in its serotonin syndrome article: "do not stop taking your medicine. Talk to your health care provider about your concerns first." [1]Urgent: serotonin syndrome is a potentially life-threatening reaction to certain medicines. [1] Signs include agitation, sweating, fever, fast heartbeat, high blood pressure, tremor and muscle spasms. [1] MedlinePlus says to contact your provider right away if you have symptoms. [1] If symptoms are severe, call 911 (US) or your local emergency number.
Why amino acids and medicines need a second look
The US Food and Drug Administration (FDA) says certain dietary supplements can change how a medicine is absorbed, broken down or cleared, and it names amino acids among the supplement types it means. [2] A 2023 review found that national health agencies had not set tolerable upper intake levels for amino acids, because well-conducted human dose-response trials were lacking. [3] That means there is no official "safe ceiling" to rely on for these products. See How medication interactions work and Interactions by drug class.How to read the "strength" column. Documented concern means a drug-information or review source we read states the interaction. Limited or indirect means the sources show a mechanism, a single case or a small study. Proper evidence not found means we looked in the places named and found nothing usable. It is not a finding of safety.
Reference table
| Supplement | Medicine type | What the sources we read say | Strength |
|---|---|---|---|
| Tryptophan, 5-HTP (5-hydroxytryptophan) | Antidepressants such as SSRIs and SNRIs, MAOIs, migraine triptans and other serotonin-affecting medicines | The Medical Letter (2012) states that taking 5-HTP or L-tryptophan in addition to an SSRI could cause serotonin syndrome. [4] MedlinePlus explains that the syndrome most often occurs when two or more serotonin-affecting medicines are taken together. [1] That page names prescription examples and does not name tryptophan or 5-HTP. A Cochrane review found only two trials of adequate quality (64 patients in total) of tryptophan or 5-HTP for depression and said a possible link with eosinophilia-myalgia syndrome had not been elucidated. [5] How often the interaction problem happens with these supplements: Proper evidence not found (we searched PubMed for tryptophan, 5-HTP, SSRI and serotonin syndrome and found no usable trial or case series). | Documented concern; frequency unknown |
| 5-HTP | Carbidopa, a medicine given with levodopa in some Parkinson's treatments | A review reports that 5-HTP combined with a peripheral decarboxylase inhibitor may cause psychopathological side effects such as acute anxiety. [6] Carbidopa is given as an example of that medicine class. [7] Levodopa-carbidopa products are described in the Parkinson's review below. [8] | Limited or indirect |
| Tryptophan, tyrosine, phenylalanine, BCAAs (leucine, isoleucine, valine) | Levodopa | A systematic review says these large neutral amino acids may compete with levodopa for the same gut and brain transport system, and that a high-protein diet can reduce levodopa's effect. It rates the overall level of evidence as low because of poor study design. [8] Trials of BCAA or other single amino acid supplements taken with levodopa: Proper evidence not found (PubMed search, 2026). | Limited or indirect (protein meals studied, not supplements) |
| Arginine (and citrulline) | Blood pressure medicines, nitrates, erectile dysfunction medicines | Arginine alone lowered blood pressure in trials: a 2011 meta-analysis of 11 placebo-controlled trials (387 people) found a fall in systolic pressure of 5.39 mm Hg [9], and a 2022 meta-analysis of 22 trials concluded it may be effective for decreasing blood pressure. [10] Whether it adds to the effect of blood pressure medicines or nitrates: Proper evidence not found in the sources we read. Our reading, not a finding of those studies, is that anyone on these medicines should tell their prescriber, because blood pressure could fall further. Citrulline: a 4-week tolerance trial in 60 healthy men reported no treatment-related adverse events [11], and says nothing about medicine interactions. | Arginine alone: documented effect on blood pressure. Interaction: Proper evidence not found |
| N-acetylcysteine (NAC) | Nitroglycerin and other nitrates | In 11 healthy volunteers, intravenous NAC potentiated the headache response to infused nitroglycerin. [12] That was an injected NAC study, not an oral supplement, so it cannot be assumed to apply to capsules. NAC became a drug before it was sold as a supplement, and the FDA treats it as excluded from the supplement definition; its 2022 initial review found no safety concerns, and a later literature review was moving to peer review. [13] See NAC and its FDA status. | Limited or indirect |
| Amino acid supplements generally (tyrosine, others) | Levothyroxine (thyroid hormone medicine) | A 2021 systematic review found limited evidence that soy products, among other foods and supplements, reduced levothyroxine absorption. [14] One published case describes a woman whose thyroid hormone levels were hard to control until a soy protein supplement was taken at a different time from her levothyroxine. [15] Tyrosine or other single amino acids with thyroid medicine: Proper evidence not found (PubMed search for tyrosine and levothyroxine, and the 2021 review itself, gave no entry for tyrosine or amino acids). | Limited or indirect (soy protein only) |
| Any single amino acid | Blood thinners, diabetes medicines, seizure medicines, cancer treatments, immune-suppressing medicines | Proper evidence not found in the sources we read. This is not a finding of safety. The FDA advises talking to your health care professional before using any dietary supplement alongside medicines. [2] | Proper evidence not found |
Medicines and conditions that raise the stakes
- Antidepressants and other serotonin-affecting medicines. The NHS page for sertraline, as one example, asks people to tell their doctor about any other medicines, herbal remedies, vitamins or supplements before starting. [16] MedlinePlus advises always telling your providers which medicines you take. [1]
- Parkinson's disease. Changes to protein intake or supplements can affect levodopa, so this is a conversation for the prescriber and a dietitian. [8]
- Heart and blood pressure medicines, thyroid medicine. Timing and combinations are set by the prescriber or pharmacist, not by a supplement label.
- Kidney or liver conditions, pregnancy, children and older adults. These groups are covered in Amino acid supplements in pregnancy, children and older adults, Protein and kidney disease and Protein and liver disease.
- Before surgery. The FDA says your health care professional may ask you to stop dietary supplements two or three weeks before a procedure, because of possible changes in heart rate, blood pressure or bleeding risk. [2] See Supplements before surgery.
Bring a list to the pharmacist
The FDA suggests bringing a list of every supplement and medicine you take to each visit. [2] Include protein powders and pre-workout products, since some contain added amino acids or caffeine. Labels and quality also matter: see Quality testing and certifications and Reading an amino acid supplement label.Frequently asked questions
Can I take tryptophan or 5-HTP with an antidepressant?
Ask the prescriber or pharmacist first. The Medical Letter states that taking 5-HTP or L-tryptophan in addition to an SSRI could cause serotonin syndrome. [4] MedlinePlus describes serotonin syndrome as potentially life-threatening, and says to contact your provider right away if you have symptoms. [1] Do not stop an antidepressant on your own.
Does arginine interact with blood pressure medicines?
Arginine on its own has lowered blood pressure in trial meta-analyses. [9][10] We did not find a study of arginine taken with blood pressure medicines or nitrates, so we cannot say how large any extra effect would be. Tell your prescriber if you use it. See Arginine, citrulline and blood pressure.
Does protein or BCAA intake affect levodopa?
A systematic review reports that a high-protein diet can reduce levodopa's effect, and that large neutral amino acids, including BCAAs, may compete with it. It rates the evidence as low. [8] Changes should be guided by the prescriber and a dietitian.
Is NAC safe with nitroglycerin?
We found one study in healthy volunteers where intravenous NAC increased the headache response to nitroglycerin. [12] Whether oral supplements do the same: Proper evidence not found. Ask a pharmacist if you use nitrates.
Where can I report a problem?
In the US, anyone can report an adverse event thought to be related to a dietary supplement to the FDA. The FDA says the first step is to contact or see your health care provider. [17] More help: Side effects, allergic reactions and reporting.
References
- MedlinePlus (US National Library of Medicine), Serotonin syndrome. Read 2026-10-23.
- US Food and Drug Administration, Mixing Medications and Dietary Supplements Can Endanger Your Health (consumer update, 2022). Read 2026-10-23.
- Elango R. Tolerable Upper Intake Level for Individual Amino Acids in Humans: A Narrative Review of Recent Clinical Studies. Adv Nutr 2023 (full text via PMC10334138). Read 2026-10-23.
- The Medical Letter on Drugs and Therapeutics, In Brief: 5-HTP for Depression, issue 1384, 20 February 2012. Read 2026-10-23.
- Shaw K, Turner J, Del Mar C. Tryptophan and 5-hydroxytryptophan for depression. Cochrane Database Syst Rev 2002. Read 2026-10-23.
- 5-Hydroxytryptophan (5-HTP): Natural Occurrence, Analysis, Biosynthesis, Biotechnology, Physiology and Toxicology. Int J Mol Sci 2021 (review). Read 2026-10-23.
- Turner EH, Loftis JM, Blackwell AD. Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan. Pharmacol Ther 2006. Read 2026-10-23.
- Wiesner A et al. How to Optimize the Effectiveness and Safety of Parkinson's Disease Therapy? A Systematic Review of Drugs Interactions with Food and Dietary Supplements. Curr Neuropharmacol 2022 (full text via PMC9881082). Read 2026-10-23.
- Dong JY et al. Effect of oral L-arginine supplementation on blood pressure: a meta-analysis of randomized, double-blind, placebo-controlled trials. Am Heart J 2011. Read 2026-10-23.
- Shiraseb F et al. Effect of l-Arginine Supplementation on Blood Pressure in Adults: A Systematic Review and Dose-Response Meta-analysis of Randomized Clinical Trials. Adv Nutr 2022. Read 2026-10-23.
- Miura N et al. Subchronic tolerance trials of graded oral supplementation with ornithine hydrochloride or citrulline in healthy adults. Amino Acids 2023 (authors employed by supplement-ingredient companies). Read 2026-10-23.
- Iversen HK. N-acetylcysteine enhances nitroglycerin-induced headache and cranial arterial responses. Clin Pharmacol Ther 1992 (intravenous N-acetylcysteine, 11 healthy volunteers). Read 2026-10-23.
- US Food and Drug Administration, FDA Releases Final Guidance on Enforcement Discretion for Certain NAC Products (constituent update, updated November 2023). Read 2026-10-23.
- Wiesner A et al. Levothyroxine Interactions with Food and Dietary Supplements: A Systematic Review. Pharmaceuticals (Basel) 2021. Read 2026-10-23.
- Bell DS, Ovalle F. Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocr Pract 2001 (single case report). Read 2026-10-23.
- NHS (UK), Taking sertraline with other medicines and herbal supplements. Read 2026-10-23.
- US Food and Drug Administration, Questions and Answers on Dietary Supplements. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.