Phenylalanine

Medically Reviewed by Melinda Ratini, MS, DO on May 30, 2023
Written by Eric Metcalf, MPH
3 min read

Phenylalanine comes in several forms as a supplement:

  • L-phenylalanine is an essential amino acid. It's also found in protein in the foods we eat.
  • D-phenylalanine
  • DL-phenylalanine, which contains both the D- and L-forms

Phenylalanine is not a widely accepted treatment for any condition. But people have tried to treat a number of conditions with phenylalanine, including:

Some research, mainly from the 1970s and 1980s, offers some support for using it for depression. Several studies also showed that L-phenylalanine plus ultraviolet A light may be helpful for people with vitiligo.

There is less evidence to support its use for other conditions.

Optimal doses of phenylalanine have not been set for any condition. Quality and active ingredients in supplements may vary widely from maker to maker. This makes it difficult to set a standard dose. 

Phenylalanine is found in most protein-containing foods, including:

  • Meat
  • Eggs
  • Fish
  • Cheese
  • Products containing aspartame

Phenylalanine can trigger allergic reactions, with symptoms such as:

Side effects may include:

Doses higher than 5,000 milligrams a day can cause nerve damage.

Risks. People with certain conditions should avoid using this supplement, including those with schizophrenia (tardive dyskinesia, a movement disorder, may develop.) You also should avoid the supplement if you have a sensitivity to phenylalanine or a condition in which your body can't break down phenylalanine such as phenylketonuria (PKU).

And use caution in taking phenylalanine if you have:

Also, it is unknown whether this supplement is safe in women who are pregnant or breastfeeding.

Interactions. Phenylalanine can cause tardive dyskinesia in people taking antipsychotic medicines.

If taken with certain antidepressants, this supplement could lead to:

Phenylalanine might also:

It may also affect how your body breaks down other drugs and supplements. And use with caution if you are taking a monoamine oxidase inhibitor (MAOI) drug and several other classes of medications; talk to your doctor or pharmacist about this.

Tell your doctor about any supplements you're taking, even if they're natural. That way, your doctor can check on any potential side effects or interactions with any medications.

The U.S. Food and Drug Administration (FDA) does regulate dietary supplements; it uses a different set of regulations than it does for "conventional" foods or drugs. Unlike drug manufacturers, the makers of supplements don’t have to show their products are safe or effective before selling them on the market.