DL-Phenylalanine

Also called DLPA, DL-Phe, D-phenylalanine, L-phenylalanine.

Best grade D across 7 graded outcomes Amino acid Last checked Oct 21, 2026

The short answer

From 12 sources

DL-phenylalanine mixes the L-form found in protein with the D-form. Evidence is old and small: a 27-person depression trial compared it only with imipramine, D-phenylalanine did not beat placebo for chronic pain, and vitiligo studies paired L-phenylalanine with UV light. People with phenylketonuria must limit phenylalanine.

Every graded outcome on this page

7 rows · line = participants

Grade DSkin repigmentation in vitiligo with UVA lightVitiligon 32
Grade DEnergy intake at a buffet meal and blood glucose after a drinkAppetiten 32
Grade DChronic pain (visual analog scale and cold pressor test)Painn 30
Grade DDepression scores vs the antidepressant imipramineMoodn 27
Grade DRemission of depression after failed antidepressantsMoodn 23
Grade DRepigmentation in childhood vitiligo with UVA lightVitiligon 13
Grade DAppetite, energy intake and gut hormones: L- vs D-phenylalanineAppetiten 11
4 more graded outcomes, in the evidence chapter

Key facts

What it is: A 50:50 mix of L-phenylalanine (the essential amino acid in food protein) and D-phenylalanine^1,8
Depression: Two small 1970s studies: one with no control group, one that compared it only with imipramine and found no difference^1,2
Pain: D-phenylalanine at 1,000 mg/day did not beat placebo in a 30-person chronic pain crossover trial³
Vitiligo: L-phenylalanine plus UVA light was linked to repigmentation in small studies; it was not tested on its own^4,5
Phenylketonuria (PKU): People with PKU cannot process phenylalanine, and high levels can damage the brain⁸

What it is

DL-phenylalanine is an equal mix of L-phenylalanine, an essential amino acid in protein, and its mirror-image D-form.

How it works

Proposed

D-phenylalanine was proposed to block the breakdown of enkephalins (the body's own opioid-like peptides) and raised pain thresholds in animals, the basis of the pain claims; a 30-person human trial found no pain relief. Oral L-phenylalanine raised blood tyrosine slightly in a vitiligo study, the rationale for its use with UV light. In the gut, large doses of L-phenylalanine release the satiety hormone CCK.^3,4,6

Forms, side by side

3 forms named in our sources. The rest show what our data holds, and say plainly where it holds nothing yet.

DL-Phenylalanine forms compared

3 forms

DL-Phenylalanine forms compared: 3 forms by 3 attributes. Empty cells are labelled No data yet, or Not graded in the evidence row.
3 forms	(DLPA)	D-phenylalanine	L-phenylalanine
Absorption in people	No head-to-head human data.	No head-to-head human data.	After an oral dose, blood L-phenylalanine peaked within 30 to 60 minutes, with a slight rise in tyrosine.⁴
GI tolerance	No important adverse reactions were reported at 50 to 100 mg/day in the 1975 preliminary note.²	The abstract of the 4-week crossover pain trial does not report adverse effects.³	No side effects were reported in 13 children given L-phenylalanine plus UVA.⁵
Studied for	Depression (small, old studies)	Chronic pain (no effect vs placebo)	Vitiligo with UV light; appetite in lab studies

DL-phenylalanine (DLPA)
Absorption in people
No head-to-head human data.
GI tolerance
No important adverse reactions were reported at 50 to 100 mg/day in the 1975 preliminary note.²
Studied for
Depression (small, old studies)
D-phenylalanine
Absorption in people
No head-to-head human data.
GI tolerance
The abstract of the 4-week crossover pain trial does not report adverse effects.³
Studied for
Chronic pain (no effect vs placebo)
L-phenylalanine
Absorption in people
After an oral dose, blood L-phenylalanine peaked within 30 to 60 minutes, with a slight rise in tyrosine.⁴
GI tolerance
No side effects were reported in 13 children given L-phenylalanine plus UVA.⁵
Studied for
Vitiligo with UV light; appetite in lab studies

What the human research says

Human trials only, one row per outcome. The form line shows what each trial actually tested. Grades follow our evidence-grading policy.

Trial map

7 graded outcomes, largest study first

7 graded outcomes: 3 improved, 3 mixed, 1 no effect. The largest, a 2020 crossover trial of 32 people on energy intake at a buffet meal and blood glucose after a drink, found mixed results.

By sizeTable

1Energy intake at a buffet meal and blood glucose after a drink

Mixed · Grade D · Crossover RCT, 32 people, 2020

Healthy lean young men (Australia), two parts of 16

Dose, result and source

Every graded outcome on this page, largest study first
Outcome	Found	People	Study	Who was studied
Energy intake at a buffet meal and blood glucose after a drink	Mixed	32	Grade D · Crossover RCT, 2020	Healthy lean young men (Australia), two parts of 16
Skin repigmentation in vitiligo with UVA light	Improved	32	Grade D · RCT, 1994	Patients with vitiligo (Netherlands); a separate open trial enrolled 149
Chronic pain (visual analog scale and cold pressor test)	No effect	30	Grade D · Crossover RCT, 1986	Adults with chronic pain of varied causes, unrelieved by earlier treatments (USA)
Depression scores vs the antidepressant imipramine	Mixed	27	Grade D · RCT, 1979	Patients with depression
Remission of depression after failed antidepressants	Improved	23	Grade D · Observational, 1975	Adults with depression that had not responded to standard antidepressants
Repigmentation in childhood vitiligo with UVA light	Improved	13	Grade D · Observational, 1989	Children with vitiligo
Appetite, energy intake and gut hormones: L- vs D-phenylalanine	Mixed	11	Grade D · Crossover RCT, 2021	Healthy volunteers, London (double-blind, placebo-controlled part)

Every graded row

7 rows

Form tested

Showing 7 of 7 outcomes. Human trials only.

Grade DSkin repigmentation in vitiligo with UVA lightVitiligoImprovedRCT
n 32 · 1994

Improved Repigmentation was reported in both trials, up to 60% in the blinded trial and 77% in the open trial; higher doses raised blood levels but not results.

Design: RCT, n = 32, 1994
Population: Patients with vitiligo (Netherlands); a separate open trial enrolled 149
Form: Oral L-phenylalanine plus UVA irradiation
Dose: Weight-based; the authors judged the best dose to be under 50 mg/kg/day
Duration: 6 months (blinded trial); 18 months (open trial)
Source: Reference 4 · PubMed 8186511 (opens PubMed)

Grade DEnergy intake at a buffet meal and blood glucose after a drinkAppetiteMixedCrossover RCT
n 32 · 2020

Mixed 10 g, but not 5 g, raised the satiety hormone CCK, cut energy intake at the next meal and lowered blood glucose after a mixed drink. Neither dose changed gastric emptying.

Design: Crossover RCT, n = 32, 2020
Population: Healthy lean young men (Australia), two parts of 16
Form: L-phenylalanine given by tube into the stomach
Dose: 5 or 10 g single dose (vs control)
Duration: Single doses on separate days
Source: Reference 6 · PubMed 32560181 (opens PubMed)

Grade DChronic pain (visual analog scale and cold pressor test)PainNo effectCrossover RCT
n 30 · 1986

No effect No significant pain relief versus placebo: 25% of patients reported more relief on D-phenylalanine, 22% on placebo and 53% no difference.

Design: Crossover RCT, n = 30, 1986
Population: Adults with chronic pain of varied causes, unrelieved by earlier treatments (USA)
Form: D-phenylalanine
Dose: 250 mg four times a day (1,000 mg/day) vs lactose placebo
Duration: 4 weeks per period
Source: Reference 3 · PubMed 3524509 (opens PubMed)

Grade DDepression scores vs the antidepressant imipramineMoodMixedRCT
n 27 · 1979

Mixed Hamilton depression and self-rated scores did not differ between DL-phenylalanine and imipramine; imipramine lowered anxiety more on days 10 and 20 and helped sleep more early on.

Design: RCT, n = 27, 1979
Population: Patients with depression
Form: DL-phenylalanine
Dose: 150 to 200 mg/day (vs imipramine)
Duration: 30 days
Source: Reference 1 · PubMed 387000 (opens PubMed)

Grade DRemission of depression after failed antidepressantsMoodImprovedObservational
n 23 · 1975

Improved The authors reported complete remission in 17 of 23 patients within 1 to 13 days and no important adverse reactions.

Design: Observational, n = 23, 1975
Population: Adults with depression that had not responded to standard antidepressants
Form: DL- or D-phenylalanine
Dose: 50 or 100 mg/day
Duration: 15 days
Source: Reference 2 · PubMed 1173765 (opens PubMed)

Grade DRepigmentation in childhood vitiligo with UVA lightVitiligoImprovedObservational
n 13 · 1989

Improved 3 children repigmented fully, 6 improved by 50 to 90% and 4 did not respond; no side effects were reported.

Design: Observational, n = 13, 1989
Population: Children with vitiligo
Form: Oral L-phenylalanine plus UVA irradiation
Dose: Not stated in the abstract
Duration: Not stated in the abstract
Source: Reference 5 · PubMed 2616391 (opens PubMed)

Grade DAppetite, energy intake and gut hormones: L- vs D-phenylalanineAppetiteNo change in food intakeCrossover RCT
n 11 · 2021

Mixed L-phenylalanine raised insulin, glucagon and GIP and lowered blood glucose after the meal vs placebo, but neither form changed hunger ratings or how much was eaten. D-phenylalanine raised the gut hormone PYY after the meal but not insulin or glucagon.

Design: Crossover RCT, n = 11, 2021
Population: Healthy volunteers, London (double-blind, placebo-controlled part)
Form: L-phenylalanine and D-phenylalanine, oral (separate arms)
Dose: 10 g single dose of each form vs placebo
Duration: Single doses on separate days; meal served 70 minutes later
Source: Reference 11 · PubMed 32991046 (opens PubMed)

What the numbers add up to

Each point is worked out from the data and the sources numbered on this page, with the arithmetic shown under it. It adds no new data and is not medical advice.

The pain claim rests on animals

D-phenylalanine raised pain thresholds in animals, but the one placebo-controlled human trial in our sources found no difference in chronic pain.³

No placebo-controlled mood trial

Neither DL-phenylalanine depression study used a placebo, and both are from the 1970s.^1,2

Studied doses

Doses used in research, shown against the official limits. They describe the studies, not a recommendation for you.

Studied doses in research, not advice. These are amounts used in research, not personal advice. Your clinician or pharmacist can say what fits you.

Doses studied in human trials

Doses studied in human trials ranged from 50 mg/day to single 10 g doses: 50 to 200 mg/day of DL- or D-phenylalanine for 15 to 30 days (depression), 1,000 mg/day of D-phenylalanine for 4 weeks per period (pain), weight-based L-phenylalanine with UVA for up to 18 months (vitiligo), and single 5 or 10 g doses of L-phenylalanine (appetite).^1,2,3,4,6

Interactions and safety

Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.

What DL-Phenylalanine meets

3 cited rows

Medicines 3

DL-Phenylalanine3 medicines · 0 supplements

Supplements and foods 0

No supplement rows in our sources.

Drug label or FDA notice
Measured in a human study

Interactions

3 agents

Published interactions, grouped by kind
Agent	What the source says	Evidence
Medicines3
MAO inhibitor antidepressantse.g. phenelzine (Nardil)	The phenelzine label says patients should not take phenylalanine, because MAO inhibitors can potentiate related compounds and cause hypertensive crises.⁹	Drug label
Levodopae.g. carbidopa/levodopa for Parkinson's disease	In Parkinson's patients, oral phenylalanine (100 mg/kg) reversed the effect of infused levodopa without lowering its blood level, consistent with competition for transport into the brain.¹⁰	PK study
Antipsychotics (in people with tardive dyskinesia)	In 18 men with schizophrenia and tardive dyskinesia, a single 100 mg/kg dose of phenylalanine increased involuntary movements versus placebo.⁷	RCT

Check with what you take

Kept on this device only

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Most cited:

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Starting from one medicine? The interaction checker rates how serious each pair is, on a curated list of 19 entries for 11 medicines. This cabinet has no severity rating: it lists every interaction row our supplement pages cite.

Who should be careful

3 groups

Phenylketonuria (PKU): People with PKU cannot break down phenylalanine; high blood levels can damage the brain, so their diet limits it.⁸
Tardive dyskinesia: In 18 men with schizophrenia and tardive dyskinesia, a single 100 mg/kg dose of phenylalanine increased involuntary movements compared with placebo.⁷
Pregnancy, breastfeeding and children: We found no trial of DL-phenylalanine in pregnancy or breastfeeding, and LactMed has no entry for it; the only pediatric study used L-phenylalanine with UV light for vitiligo. Women with PKU must keep phenylalanine low before and throughout pregnancy, because high levels damage the developing baby.^1,3,5,12

Reading the label, and the cost per dose

Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.

How to read the label

2 checks

DL vs L vs D

DLPA is half D-phenylalanine, the form tested for pain, and half L-phenylalanine, the form in food and the vitiligo studies. Results for one form do not automatically apply to the mix.^3,4

Label dose vs studied dose

The DL-phenylalanine depression studies used 50 to 200 mg/day; the pain trial used 1,000 mg/day of the D-form.^1,2,3

Have the bottle? Photograph its Supplement Facts panel and see each line, dl-phenylalanine included, beside the doses and forms the trials used in Label X-Ray.

Cost per studied dose, without the sales pitch

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Price paid ($)Servings per containerDL-phenylalanine per serving (mg)

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Frequently asked questions

Short answers to common questions; the numbers link to the sources below.

Common questions

8 answers

Does DL-phenylalanine help depression?

The evidence is two small 1970s studies. One had no control group; the other found no difference from imipramine, which cannot show either worked because there was no placebo.^1,2

Does DL-phenylalanine relieve chronic pain?

In a 30-person double-blind crossover trial, D-phenylalanine at 1,000 mg/day did not relieve chronic pain more than placebo.³

Does phenylalanine help vitiligo?

Small studies combined oral L-phenylalanine with UVA light and reported repigmentation, but the abstracts do not show its effect apart from the light treatment.^4,5

Does phenylalanine reduce appetite?

In a lab study, a single 10 g dose given by stomach tube raised the satiety hormone CCK and cut intake at the next meal in 16 healthy men; 5 g did not.⁶

Do D- and L-phenylalanine act differently?

Yes, in one 11-person crossover study a single 10 g dose of L-phenylalanine raised insulin, glucagon and GIP, while D-phenylalanine did not; D-phenylalanine raised PYY instead. Neither changed hunger or food intake.¹¹

Who should not use phenylalanine?

People with phenylketonuria (PKU) must limit phenylalanine. A single large dose also worsened involuntary movements in men with tardive dyskinesia.^7,8

What are the side effects of DL-phenylalanine?

Side-effect data are thin. A 1975 report found no important adverse reactions at 50 to 100 mg/day, and single 10 g doses of L-phenylalanine were tolerated in a small lab study. A single 100 mg/kg dose worsened involuntary movements in men with tardive dyskinesia.^2,11,7

Can DL-phenylalanine be combined with antidepressants?

Not with MAO inhibitors: the phenelzine (Nardil) label says patients should not take phenylalanine because of the risk of hypertensive crises. The 1979 trial compared DL-phenylalanine with imipramine rather than combining them, so there are no trial data on combinations.^9,1

Sources

12 sources: trial abstracts, NIH fact sheets and reference databases. Numbers in the text link here.

Sources cited on this page

12 sources

1DL-phenylalanine versus imipramine: a double-blind controlled study.RCT Arch Psychiatr Nervenkr (1970), 1979 · PMID 387000
2Therapy of depression by phenylalanine. Preliminary note.Observational Arzneimittelforschung, 1975 · PMID 1173765
3Analgesic effectiveness of D-phenylalanine in chronic pain patients.Crossover RCT Arch Phys Med Rehabil, 1986 · PMID 3524509
4L-phenylalanine and UVA irradiation in the treatment of vitiligo.RCT Dermatology, 1994 · PMID 8186511
5Phenylalanine plus ultraviolet light: preliminary report of a promising treatment for childhood vitiligo.Observational Pediatr Dermatol, 1989 · PMID 2616391
6Effects of L-Phenylalanine on Energy Intake and Glycaemia-Impacts on Appetite Perceptions, Gastrointestinal Hormones and Gastric Emptying in Healthy Males.Crossover RCT Nutrients, 2020 · PMID 32560181
7Tardive dyskinesia exacerbated after ingestion of phenylalanine by schizophrenic patients.Crossover RCT Neuropsychopharmacology, 1997 · PMID 9015796
8Phenylketonuria (PKU)Fact sheet MedlinePlus, U.S. National Library of Medicine, 2026 · medlineplus.gov
9NARDIL (phenelzine sulfate) tablets: prescribing informationLabel DailyMed, U.S. National Library of Medicine, 2025 · dailymed.nlm.nih.gov
10The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport.PK study N Engl J Med, 1984 · PMID 6694694
11Differential effects of L- and D-phenylalanine on pancreatic and gastrointestinal hormone release in humans: A randomized crossover study.Crossover RCT Diabetes Obes Metab, 2021 · PMID 32991046
12PhenylketonuriaFact sheet MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine, 2025 · medlineplus.gov

What changed

Every dated change to this page: what changed, who made it and why.

Change log

4 entries

Dated changes to this page
Date	Change	Who and why
Oct 21, 2026evidence, who should avoid, FAQ, questions, related	Added a 2021 double-blind crossover study comparing single 10 g doses of L- and D-phenylalanine with placebo (appetite and gut hormones, grade D). Rewrote the pregnancy note with MedlinePlus guidance on PKU in pregnancy and noted that LactMed and LiverTox have no phenylalanine entry. Added FAQ items on side effects, antidepressant combinations and D- vs L-forms, page questions, and related links (L-tryptophan, stress goal hub). Searched PubMed for 2023–2026 meta-analyses and RCTs of DL-, D- or L-phenylalanine supplements: none qualified (results were PKU, depletion and metabolomics studies). Checked interactions: no new ones found beyond MAO inhibitors, levodopa and antipsychotics; a 2007 study found L-phenylalanine did not change cefdinir levels (not added). Not added: a 6-person exercise fat-oxidation trial (PMID 28919843; too small).	Writer: Kymata Health EditorialTrigger: A–Z audit against docs/supplements/AUDIT-STANDARD.md
Oct 17, 2026safety	Independent review: added drug interactions for MAO inhibitors (phenelzine label, DailyMed), levodopa (PubMed-verified NEJM study) and antipsychotics in tardive dyskinesia.	Writer: Kymata Health EditorialTrigger: review
Oct 17, 2026all	First draft. Every PMID checked against NCBI E-utilities esummary (title and year taken from the record) and abstracts read for every cited row.	Writer: Kymata Health EditorialTrigger: new page
Oct 17, 2026all	Adversarial review pass: every evidence row's numbers machine-checked against its PubMed abstract, retraction and erratum flags checked for all PMIDs, unverified country and affiliation details removed, and claims not supported by the cited abstracts reworded or dropped.	Writer: Kymata Health EditorialTrigger: review

Supplement & Nutrient Encyclopedia. Educational information only; it is not medical advice and does not replace a clinician. Dietary supplements are not intended to diagnose, treat, cure or prevent any disease. Statements about supplements have not been evaluated by the FDA.

DL-Phenylalanine

Medicines 3

Supplements and foods 0

MAO inhibitor antidepressants

Antipsychotics (in people with tardive dyskinesia)

Levodopa