Overview

Phenylalanine is an amino acid that is naturally found in foods and in the artificial sweetener, aspartame. Supplements containing phenylalanine are used for various reasons, but their benefits are not well defined. Common side effects include headache and upset stomach. Before taking phenylalanine, make sure to tell your health care provider about any health conditions you have or medicines you are taking. People who have a rare inherited condition called phenylketonuria should avoid products that contain phenylalanine.

Uses

What is phenylalanine used for?

Phenylalanine is used for many reasons. However, its benefits are not well defined. Some common reasons for using phenylalanine include the following.

• To improve brain health, including memory and thinking skills
• To improve mood
• To lower blood sugar levels
• To reduce appetite

Note: The FDA has not reviewed phenylalanine for safety and effectiveness. Learn more about dietary supplements here.

How does phenylalanine work (mechanism of action)?

Phenylalanine is an amino acid, which is a building block of protein. Your body is not able to make phenylalanine on its own, so you must get it from the foods in your diet. Many foods contain phenylalanine, including meats, seafood, eggs, soy, nuts, seeds, and cheese. It is also found in aspartame, a common artificial sweetener. Your body uses phenylalanine to build proteins and to make another amino acid called tyrosine.

When phenylalanine is eaten as part of the diet, it may play a role in telling your body to produce certain hormones, such as insulin and glucagon. This may help reduce blood sugar levels after a meal by a small amount. But the benefits of taking phenylalanine to reduce either appetite or blood sugar are not well defined.

Over the years, there has been interest in taking phenylalanine for certain neurological disorders, such as Parkinson’s disease or attention deficit hyperactivity disorder (ADHD). This is based on a theory that people with these conditions may have low levels of phenylalanine. But there is no good evidence to support this theory, and the benefits of phenylalanine for brain health are not well defined.

How should I store phenylalanine?

You should store phenylalanine according to the product label. Most dietary supplements should be kept in a cool, dry location that is out of the reach of children and pets and not in direct sunlight.

Side Effects

What are the most common side effects of phenylalanine?

The most common side effects of phenylalanine are listed below. These side effects may vary depending on the dose you are taking or the specific product you are using. Tell your health care provider if you have any of these side effects that bother you.

• Headache
• Upset stomach

There may be other side effects of phenylalanine that are not listed here. Contact your health care provider if you think you are having a side effect of a supplement. In the U.S., you can report side effects to the FDA at www.safetyreporting.hhs.gov or by calling 888-SAFEFOOD (888-723-3366). In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of phenylalanine?

While less common, the most serious side effects of phenylalanine are described below, along with what to do if they happen.

Severe Allergic Reactions. Phenylalanine may cause allergic reactions, which can be serious. Stop taking phenylalanine and get help right away if you have any of the following symptoms of a serious allergic reaction.

• Breathing problems or wheezing
• Racing heart
• Fever or general ill feeling
• Swollen lymph nodes
• Swelling of the face, lips, mouth, tongue, or throat
• Trouble swallowing or throat tightness
• Itching or skin rash
• Bumps on the skin called hives that can be red, pink, white, or brown depending on your skin tone
• Nausea or vomiting
• Dizziness, feeling lightheaded, or fainting
• Stomach cramps
• Joint pain

Precautions

Who should not use phenylalanine?

Allergies to Ingredients. People who are allergic to any of the following should not take phenylalanine.

• Phenylalanine
• Any of the ingredients in the specific product

Your pharmacist can tell you all of the ingredients in the specific phenylalanine products they stock.

Phenylketonuria.People with a rare, inherited condition called phenylketonuria (PKU) are not able to break down phenylalanine, which can lead to serious health problems. If you have phenylketonuria, you should not take products that contain phenylalanine.

What should I know about phenylalanine before using it?

Phenylalanine is a dietary supplement. It cannot be marketed to treat or cure a disease.

Read and follow the directions on the specific product you are taking, or take it as recommended by your health care provider.

Keep phenylalanine out of the reach of children.

Supplements that contain phenylalanine may contain various forms, including D-phenylalanine, L-phenylalanine, and D,L-phenylalanine. L-phenylalanine is the form naturally found in proteins and foods. D-phenylalanine and D,L-phenylalanine are made in a lab. Each of these forms may have slightly different effects in the body.

The FDA has not tested phenylalanine products to confirm that they contain the ingredients stated on their labels. Some dietary supplements have been tested by third-party organizations to confirm that they contain the ingredients listed on their labels and do not contain any harmful chemicals. Learn more about how to choose a safe dietary supplement product here.

What should I tell my health care provider before using phenylalanine?

Tell your health care provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them know if phenylalanine is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions.Tell your health care provider if you have any of the following.

• Kidney problems
• Liver problems

Other Medicines and Supplements.Phenylalanine may interact with other medicines and supplements. Before taking phenylalanine, tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how phenylalanine could affect pregnancy or harm a fetus. Tell your health care provider if you are pregnant or plan to become pregnant.

Breastfeeding. Phenylalanine passes into breast milk. Tell your health care provider if you are breastfeeding or plan to breastfeed.

Interactions

Does phenylalanine interact with foods or drinks?

There are no known interactions between phenylalanine and foods or drinks.

It is unknown if drinking alcohol will affect phenylalanine.

Does phenylalanine interact with medicines (drug interactions)?

Always tell your health care provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking phenylalanine.

• A monoamine oxidase inhibitor (MAOI) within the past two weeks, which is a medicine usually used for depression or Parkinson’s disease
• Levodopa, which is a medicine for Parkinson’s disease

The interactions between phenylalanine and medicines are not fully understood. As with most dietary supplements, the research on drug interactions with phenylalanine is incomplete. Always check with your health care provider before taking phenylalanine with any medicines.

Dosing

What should I do if I accidentally use too much phenylalanine?

If you or someone else has used too much phenylalanine, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

Read the product label closely, and speak with a health care provider about how to take phenylalanine safely. Do not take double or extra doses, even if you miss a dose.