Phenylalanine
Sold for mood, pain and focus. In small ADHD trials it either did nothing or helped briefly and then stopped working.
At a glance
| Our verdict | Did nothing or helped briefly in small ADHD trials; dangerous in phenylketonuria and with MAOI antidepressants. |
| Sold for | Mood and ADHD (claimed) |
| How sure the evidence is | Very low, and negative |
| Cost a month | About $15 to $25 (estimated) |
Written for adults in Canada. It explains the evidence; it is not advice for you personally. Last reviewed 20 October 2026.
Supplement Check
Is it right for you?
Phenylalanine is dangerous with some antidepressants and in phenylketonuria. Your check looks at every medicine you take and tells you what is worth doing for focus. Our Supplement Check looks at:
- Phenylketonuria (PKU): never
- Pregnancy in anyone with high phenylalanine levels
- An MAOI now or in the last 14 days
- Bipolar disorder
- Melanoma (a theoretical concern)
- Every medicine you take
Coming soon. The Supplement Check and the Online Health Assessment open once our safety testing is complete.
$59 when it opens, paid once. Adults 18 and over, anywhere in Canada. While the service is new, Dr. French reads every report himself, and your report arrives within 3 working days. More about the Supplement Check
What it is
Phenylalanine is an amino acid. The L-form is essential and comes from food protein; the D-form is made in the laboratory; DL-phenylalanine is a mixture of the two.
How it acts
The body turns L-phenylalanine into tyrosine and then into dopamine and noradrenaline, the nerve messengers that ADHD medicines act on. D-phenylalanine slows the breakdown of the body's own pain-relieving chemicals (enkephalins) and is turned into phenylethylamine. That is mechanism, not proof of benefit.
What it is used for, and what the evidence says
| Use | What the trials found | How sure we are |
|---|---|---|
| Adults with ADHD, DL-phenylalanine | 19 adults (13 finished), 2-week double-blind crossover: overall improvement only came close to significance; mood improved; those who responded lost all benefit within 3 months. [1] | Very low |
| Adults with ADHD, L-phenylalanine | An open trial (no placebo) found no effect. [1] | Very low |
| Boys with hyperactivity, D-phenylalanine | 11 boys, 20 mg per kg, crossover: no effect. [2] | Very low |
How sure we are: High means further research is very unlikely to change the answer; Moderate, it might; Low, it may well change; Very low, we are very unsure. These follow the GRADE method that guideline panels use.
Benefits
- None established in ADHD.
Risks and side effects
- Dangerous in phenylketonuria (PKU).
- High blood pressure with MAOI antidepressants.
- Anxiety, headache.
- Tardive dyskinesia (involuntary movements) has been reported with antipsychotic medicines (case reports).
Interactions with medicines
Our review found no report that it speeds up or blocks the liver enzymes that clear medicines; that question has not been studied for it.
| Medicine | What can happen | What to do |
|---|---|---|
| MAOI antidepressants and related medicines, now or in the last 14 days: phenelzine, tranylcypromine, moclobemide, selegiline, rasagiline, safinamide, linezolid, methylene blue | Dangerous rise in blood pressure | Do not take |
| Levodopa (for Parkinson disease) | Competes with it | Do not take |
| Antipsychotic medicines | Involuntary movements reported (case reports) | Do not take |
| ADHD stimulants and atomoxetine | Adds to the same nerve messengers; never tested | Do not take |
Who should not take it
- Phenylketonuria (PKU): never.
- Pregnancy in anyone with high phenylalanine levels.
- An MAOI now or in the last 14 days.
- Bipolar disorder.
- Melanoma (a theoretical concern).
- Anyone on a stimulant or atomoxetine.
- Under 18: outside the Supplement Check.
Dose and timing
We do not recommend a dose. For reference, the trial of D-phenylalanine in boys used 20 mg per kg of body weight; the adult DL-phenylalanine trial ran for 2 weeks.
What we know about Canadians
Canada has no measured national figure for ADHD (attention deficit hyperactivity disorder) in adults. Worldwide, about 2.6% of adults have ADHD that began in childhood and 6.8% have the symptoms. [3] Diagnosis and treatment in adults have risen sharply: in Ontario, new stimulant prescriptions rose 157% from 2015 to 2023, with the steepest rises in women aged 25 to 44 (up 421%) and 18 to 24 (up 369%). [4] In British Columbia, new adult diagnoses went from about 9 to 35 per 100,000 a month between before and after the pandemic. [5]
Most at risk of being newly diagnosed or treated: women aged 18 to 44. With so many more adults on stimulant medicines, what is taken alongside them matters more.
No Canadian data on phenylalanine supplement use in our review.
How sure are we: low. Low for how many Canadian adults have ADHD (a global estimate); high for the rise in prescribing (whole-population Ontario and British Columbia data).
An honest comparison: adult ADHD symptoms
Effect sizes are put on one common scale so that different symptom questionnaires can be compared: the bigger the number, the more symptoms fell compared with a dummy pill (placebo). The medicine figures come from blinded trials in adults, rated by clinicians at about 12 weeks; the self-rated figures are closer to what you would notice yourself.
| Option | Effect | How sure we are | Cost a month (CAD) |
|---|---|---|---|
| Phenylalanine (any form) | No effect, or a brief effect that wore off [1, 2] | Very low | $15 to $25 (estimated) |
| Aerobic exercise, 20 to 45 minutes most days | Core symptoms improved a small amount after a single session (effect size 0.23); results of longer programmes were mixed [6] | Low | $0 to $60 (gym) |
| Lisdexamfetamine and other amphetamines | Clinician-rated effect size 0.79, the largest of any treatment [7]; self-rated 0.39 (stimulants as a group) [8] | Moderate to High | $90 to $150 (estimated; generic lower) |
| Methylphenidate, long-acting | Clinician-rated effect size 0.49 [7] | Moderate to High | $45 to $75 (estimated) |
| Atomoxetine (non-stimulant) | Clinician-rated effect size 0.45 [7]; self-rated 0.38 [8] | Moderate | $45 to $60 (estimated) |
Why we don't recommend it
We do not sell phenylalanine and our check never recommends it.
Already taking it? Stop it, and tell your doctor at once if you take an MAOI antidepressant or have phenylketonuria. If you take a stimulant or atomoxetine: check your blood pressure and pulse within 2 weeks of starting any new supplement, and tell your prescriber about every supplement you take. If a questionnaire suggests ADHD, that is a screen, not a diagnosis: it needs a formal assessment. If ADHD is confirmed, the prescription medicines reduce symptoms two to four times more than any supplement.
In small ADHD trials phenylalanine either did nothing or helped briefly and then stopped working. [1, 2] It is dangerous for anyone with phenylketonuria and can raise blood pressure with MAOI antidepressants. We do not sell it, and we suggest you do not take it.
Supplement Check
Before you buy it, get it checked
Phenylalanine is dangerous with some antidepressants and in phenylketonuria. Your check looks at every medicine you take and tells you what is worth doing for focus. Our Supplement Check looks at:
- Phenylketonuria (PKU): never
- Pregnancy in anyone with high phenylalanine levels
- An MAOI now or in the last 14 days
- Bipolar disorder
- Melanoma (a theoretical concern)
- Every medicine you take
Coming soon. The Supplement Check and the Online Health Assessment open once our safety testing is complete.
$59 when it opens, paid once. Adults 18 and over, anywhere in Canada. While the service is new, Dr. French reads every report himself, and your report arrives within 3 working days. More about the Supplement Check
Disclosure of financial interest
White Pine Medical earns money from products bought through our Fullscript dispensary. Dr. French's Code of Ethics requires him to tell you this. You do not have to buy anything from us: every product is named by its ingredient and dose so you can buy it at any pharmacy, and the advice is the same wherever you buy it, or if you buy nothing.
References
- Wood DR, Reimherr FW, Wender PH. Treatment of attention deficit disorder with DL-phenylalanine. Psychiatry Res. 1985;16(1):21-6. PMID 3903813.
- Zametkin AJ, Karoum F, Rapoport JL. Treatment of hyperactive children with D-phenylalanine. Am J Psychiatry. 1987;144(6):792-4. PMID 3296793.
- Song P, Zha M, Yang Q, Zhang Y, Li X, Rudan I, et al. The prevalence of adult attention-deficit hyperactivity disorder: a global systematic review and meta-analysis. J Glob Health. 2021;11:04009. PMID 33692893.
- Myran DT, MacDonald-Spracklin R, Busa G, Talarico R, Finkelstein Y. Population-level trends in attention-deficit/hyperactivity disorder medication prescribing. JAMA Netw Open. 2025;8(12):e2548532. PMID 41379449.
- Hu K, Desai R, Au S, Zhao B, Barbic S, Marchand K, et al. Trends of incident adult attention-deficit/hyperactivity disorder diagnoses before, during and after the pandemic provincial state of emergency in British Columbia (2013-2023): a population-based study. Lancet Reg Health Am. 2025;51:101223. PMID 41282502.
- Xu S, Zhao C, Hu L. The effects of acute and chronic exercise on executive functions and core symptoms in adults with ADHD: a systematic review and meta-analysis. Psychol Sport Exerc. 2026;84:103088. PMID 41638541.
- Cortese S, Adamo N, Del Giovane C, et al. Comparative efficacy and tolerability of medications for attention-deficit hyperactivity disorder in children, adolescents, and adults: a systematic review and network meta-analysis. Lancet Psychiatry. 2018;5(9):727-38. PMID 30097390.
- Ostinelli EG, Schulze M, Zangani C, et al. Comparative efficacy and acceptability of pharmacological, psychological, and neurostimulatory interventions for ADHD in adults: a systematic review and component network meta-analysis. Lancet Psychiatry. 2025;12(1):32-43. PMID 39701638.
Costs marked as estimates are typical Canadian retail or pharmacy prices, before any drug plan. Certainty grades are ours unless the review gave its own.