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Medically reviewed by Last updated on Feb 12, 2024.

Excipient (pharmacologically inactive substance)

What is it?

Phenylalanine is one of the common amino acids in proteins, and a nutritionally essential amino acid. Phenylalanine occurs naturally in many protein-rich foods, such as milk, eggs and meat. Phenylalanine also is sold as a dietary supplement. For people who have the genetic disorder phenylketonuria (PKU), phenylalanine can be a serious health concern. Phenylalanine can cause mental retardation, brain damage, seizures and other problems in people with PKU. The artificial sweetener aspartame (Equal, NutraSweet), which is added to many medications, diet foods and diet sodas, contains phenylalanine. Federal regulations require that any food that contains aspartame bear this warning: "Phenylketonurics: Contains phenylalanine." This warning helps people with PKU avoid products that are a source of phenylalanine.[1]

List of medications using Phenylalanine


  1. Mayo Clinic FAQs: Phenylalanine in diet soda: Is it harmful? Accessed February 27, 2015 at

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.