Phenylalanine and its side effects

Phenylalanine is an essential amino acid or building block for proteins. If you use more than the recommended amount, side effects such as itching, breathing problems, anxiety, nervousness and hyperactivity may occur, especially in people with certain health conditions. That is why it is important to know in which products this substance is present.

Where do we find phenylalanine?

To avoid the possible side effects of phenylalanine, it is essential to know which products contain this substance.

Natural and synthetic phenylalanine

Naturally occurring phenylalanine is called L-phenylalanine and the synthetic variant or the one created by humans themselves in a lab is called D-phenylalanine. A mixture of both then becomes DL-phenylalanine.

Phenylalanine is essential for the body, but it is important that you get it from a natural source rather than from supplements. Examples of foods that contain natural phenylalanine are milk, yogurt, cottage cheese, bananas, nuts, meat and poultry.


The artificial sweetener aspartame contains quite large amounts of phenylalanine. Therefore, it is also mandatory to label foods containing aspartame or phenylalanine with: “phenylketonuria: Contains phenylalanine”. Aspartame is used in more than 6,000 foods including diet sodas, sugar-free drinks, chewing gum, mints, low-calorie yogurt and ice cream, sugar-free snacks and artificial sweeteners. Some medicines and personal care products may also contain aspartame.

Recommended quantity

The recommended daily allowance of phenylalanine is approximately 15 mg per kilogram of body weight. People who consume many artificially sweetened drinks and foods on a daily basis can easily exceed the recommended daily allowance, resulting in unwanted side effects.

Possible side effects

Although phenylalanine is necessary for optimal health, an overdose of this substance is not a good thing and can lead to certain side effects. Adverse reactions can also occur when phenylalanine is combined with certain medications. If you are considering taking phenylalanine supplements, it is best to discuss this with a doctor who can make you aware of the possible side effects.

Allergic reaction

Some people may experience an allergic reaction to too much phenylalanine. Signs of an allergic reaction may include:

  • Itch
  • Swelling of the hands or feet
  • Difficulty breathing
  • Mouth tingling

Anxiety, nervousness or hyperactivity

Children may experience increased levels of anxiety, nervousness or hyperactivity after taking DL-phenylalanine.


Phenylketonuria occurs when a person is born without the ability to adequately break down phenylalanine. This can cause an increased level of phenylalanine in the body:

  • Reduced learning ability
  • Larger head circumference than normal
  • Hyperactivity
  • Jerking movements
  • Epileptic attacks
  • Pigmentation problems in the skin and hair

Increased risk during pregnancy

Increased levels of phenylalanine in the body during pregnancy can increase the risk of developmental disorders, growth delays, birth defects and microencephaly. This makes it important that pregnant women with phenylketonuria receive very clear nutritional advice. Since the results of the use of aspartame are still quite controversial, it is best to consult your doctor about the risks if you are pregnant or breastfeeding.

High bloodpressure

Research has shown that the combination of phenylalanine and monoamine oxidase inhibitors (anti-depressants) can lead to dangerously high blood pressure, which can lead to a stroke or heart attack. To avoid additional adverse side effects, the use of phenylalanine is not recommended if you take the following medicines:

  • Baclofen (Lioresal)
  • Levodopa (Sinemet)
  • Anti-psychotic medications including Dilantin, Depakene and Tegretol

Digestive disorders and headaches

Phenylalanine can cause digestive disorders such as heartburn, nausea and constipation as well as headaches and dizziness, especially when taken in the form of aspartame. These side effects are caused by the rapid increase in phenylalanine levels in the brain that occurs when a large amount of aspartame is consumed.

Use phenylalanine with caution

Phenylalanine should always be consumed in moderation by healthy individuals. For most people, this consumption remains within the recommended range if you consistently use artificial sweeteners or artificially sweetened drinks. It is mainly due to daily use of these products that you can experience unwanted symptoms. If you do need to take phenylalanine to treat a medical condition, it is best to always do this in consultation with a doctor so that you are aware of the possible development of side effects.

Similar Posts