Who should not take L-tyrosine?
L-tyrosine is a popular supplement for focus under stress, but because it feeds into dopamine, adrenaline and thyroid hormone production it is not suitable for everyone. Our editorial team sets out who should avoid it and why.
What L-tyrosine is and why people take it
L-tyrosine is a non-essential amino acid: the body makes it from phenylalanine, and it is also present in protein-rich foods such as cheese, chicken, fish, soya, eggs and seeds. Tyrosine is the starting point for the synthesis of dopamine, noradrenaline and adrenaline – the catecholamines – and for thyroid hormones and the pigment melanin.
As a supplement, tyrosine is promoted for focus, motivation and resilience under stress. The research supports a narrower claim. A review by Jongkees and colleagues in 2015 concluded that tyrosine may help maintain cognitive performance in demanding situations such as sleep deprivation, cold exposure or multitasking, when catecholamine stores may be depleted, but offers little benefit under normal conditions.
Studies in military cadets, such as that by Deijen and co-workers in 1999, and in cold and altitude stress by Banderet and Lieberman, form much of this evidence. For exercise performance, results are inconsistent; Tumilty and colleagues found improved endurance capacity in the heat in one study, while others found no effect.
Because tyrosine affects neurotransmitter and hormone pathways, it is not suitable for everyone. Knowing who should avoid it is more important than any potential benefit.
Thyroid conditions
Tyrosine is a building block of thyroxine (T4) and triiodothyronine (T3). In theory, extra tyrosine could increase thyroid hormone production, although in people with adequate iodine and normal thyroid function this effect seems limited.
People with hyperthyroidism (an overactive thyroid) or Graves' disease should avoid tyrosine supplements unless their specialist agrees. Those taking levothyroxine for an underactive thyroid should also consult their doctor, because any change in hormone balance could make dose adjustment harder and confuse blood test results.
Medicines that interact
Several medicines can interact with tyrosine in ways that matter clinically.
| Medicine | Possible interaction | Advice |
|---|---|---|
| MAO inhibitors (e.g. phenelzine, tranylcypromine, selegiline) | Excess catecholamines; risk of dangerous rise in blood pressure | Avoid tyrosine |
| Levodopa (Parkinson's disease) | Competes for absorption and brain uptake; may reduce effectiveness | Avoid unless neurologist agrees |
| Thyroid hormones (levothyroxine, liothyronine) | Possible additive effect on thyroid activity | Discuss with GP |
| Stimulants and ADHD medication | Possible additive effect on heart rate and blood pressure | Discuss with prescriber |
The interaction with MAO inhibitors is the most serious. These antidepressants prevent the breakdown of monoamines. People taking them must already follow dietary restrictions on tyramine-rich foods, and adding a precursor of noradrenaline is not advisable.
Levodopa is relevant because tyrosine and levodopa share transport systems in the gut and across the blood–brain barrier. Large amounts of competing amino acids can reduce the effect of levodopa, which is why neurologists sometimes advise patients to time protein intake around their doses.
Other groups who should be cautious
People with a history of melanoma are often advised to avoid tyrosine supplements because tyrosine is a precursor of melanin. Evidence that supplements affect melanoma progression is lacking, but the theoretical concern has led to a cautious approach.
- Pregnancy and breastfeeding: safety of supplemental doses not established.
- Migraine: some individuals report that tyrosine or catecholamine-raising products trigger headaches.
- High blood pressure or heart rhythm problems: stimulant-like effects possible, particularly with caffeine.
- Anxiety disorders and insomnia: some users report restlessness or difficulty sleeping.
- Inherited disorders of tyrosine metabolism (tyrosinaemia): supplements contraindicated; diet managed by specialists.
Phenylketonuria (PKU) is a special case. People with PKU cannot convert phenylalanine to tyrosine effectively, and tyrosine is supplied as part of medically supervised protein substitutes. Any changes to tyrosine intake must be managed by the metabolic team rather than by buying a separate supplement.
Readers frequently ask us about Drostanolone Tablets.
Competing athletes should note that tyrosine is not on the WADA Prohibited List, but many pre-workout products that contain it also include stimulants that may be. Choose batch-tested single-ingredient products if needed.
Sensible use for healthy adults
For healthy adults without the conditions above, tyrosine is generally well tolerated. Studies on cognition under stress typically used amounts based on body weight, often around 100–150 mg per kg taken before a demanding task, which is much higher than the 500–1000 mg found in typical capsules. Follow the label of a reputable product rather than trying to reproduce research amounts.
Possible side effects include nausea, heartburn, headache and restlessness. Taking it in the morning or early afternoon avoids sleep problems. Combining it with high caffeine intake may increase jitteriness and heart rate.
Remember that a diet with enough protein already supplies tyrosine in quantity. Supplements are optional and situational, not a daily necessity.
Editorial team's conclusions
L-tyrosine may help maintain cognitive performance under acute stress, but its benefits in normal conditions are limited.
It should be avoided by people taking MAO inhibitors, those with hyperthyroidism and, unless a specialist agrees, those taking levodopa or with a history of melanoma. Thyroid medication, stimulants and pregnancy also call for medical advice.
For healthy adults, it is usually well tolerated at label amounts, but it remains an optional supplement rather than a necessity.
You may also find our articles on caffeine and pre-workout stimulants, on thyroid function in athletes and on nootropics for students who train useful.
References
- Jongkees BJ, Hommel B, Kühn S, Colzato LS. Effect of tyrosine supplementation on clinical and healthy populations under stress or cognitive demands – a review. J Psychiatr Res. 2015;70:50–57.
- Deijen JB, Wientjes CJ, Vullinghs HF, et al. Tyrosine improves cognitive performance and reduces blood pressure in cadets after one week of a combat training course. Brain Res Bull. 1999;48(2):203–209.
- Banderet LE, Lieberman HR. Treatment with tyrosine, a neurotransmitter precursor, reduces environmental stress in humans. Brain Res Bull. 1989;22(4):759–762.
- Tumilty L, Davison G, Beckmann M, Thatcher R. Oral tyrosine supplementation improves exercise capacity in the heat. Eur J Appl Physiol. 2011;111(12):2941–2950.
- Fernstrom JD, Fernstrom MH. Tyrosine, phenylalanine, and catecholamine synthesis and function in the brain. J Nutr. 2007;137(6 Suppl 1):1539S–1547S.
