What is phenylketonuria?

Phenylketonuria (PKU) is a genetic (inherited) condition in which levels of phenylalanine build up in the blood. Phenylalanine is an amino acid; amino acids are the building blocks of proteins. Many food protein sources contain phenylalanine.

If it is not treated, PKU causes serious complications, such as intellectual disabilities. In the United States, one in 10,000 to 15,000 babies is born with PKU every year.

Who is at risk of developing phenylketonuria?

While anyone can develop PKU, certain ethnic groups are more at risk, especially people of Native American or European descent.

Symptoms and Causes

What causes phenylketonuria?

PKU is caused by the buildup of phenylalanine in the body. Your body uses this amino acid to make other proteins. If you have PKU, your body can’t break down phenylalanine. If it is not treated, this buildup damages the brain and causes PKU symptoms.

What are the symptoms of phenylketonuria?

A child who is born with PKU and is not treated will have intellectual and developmental problems. He or she will also have less pigmentation in the skin.

People living with classic PKU (also called severe PKU) and who are not on a metabolic diet or being treated have the following symptoms:

  • Seizures or shaking
  • Eczema
  • Skin and hair discoloration
  • Microcephaly (small head size)
  • A musty odor to the breath, skin or urine (caused by extra phenylalanine)
  • Heart defects

Children born with moderate or mild cases of PKU may have less-severe intellectual disabilities. Some children with mild PKU don’t have any symptoms.

Diagnosis and Tests

How is phenylketonuria diagnosed?

PKU is diagnosed with a blood test. In the United States and most other countries, a blood test is taken through a heel stick on newborn babies within 48 hours of birth. Further tests will be required to confirm the type of PKU and plan the best way of treating it. In older people, PKU is diagnosed with a blood test that measures levels of phenylalanine in the blood.

Management and Treatment

How is phenylketonuria treated?

PKU is treated by cutting back on protein to limit the intake of phenylalanine. If you or your child is diagnosed with moderate to severe PKU, you will need to adjust the diet to include special formulas and meals. These changes include a diet that is low in phenylalanine, while ensuring adequate protein intake. It’s also important to avoid the artificial sweetener aspartame, which raises the levels of phenylalanine in the body.

A dietitian who is well versed in PKU can help create a diet for you. You must follow this diet for the rest of your life. As part of treatment, you’ll need frequent blood tests to monitor your phenylalanine levels.

If you have PKU and are pregnant, your doctor and dietitian will create a diet plan that ensures you receive proper nutrition while lowering your baby’s risk for complications.

Some people who have PKU also benefit from the medication sapropterin dihydrochloride (Kuvan®), which helps break down phenylalanine.


Can phenylketonuria be prevented?

There is no way to prevent PKU.

Outlook / Prognosis

What is the prognosis (outlook) for people who are treated for phenylketonuria?

With lifelong treatment, most people living with PKU lead healthy lives.

Living With

When should I call my doctor about phenylketonuria?

If you or your child develops symptoms of phenylketonuria, contact your doctor for a thorough evaluation. Ask your doctor about PKU testing before becoming pregnant or during your first prenatal examination. If you have PKU, regular blood tests monitor your condition.

Last reviewed by a Cleveland Clinic medical professional on 10/18/2018.


  • National Human Genome Research Institute. Learning About Phenylketonuria (PKU). ( Accessed 10/19/2018.
  • U.S. National Library of Medicine. Genetics Home Reference. Phenylketonuria. ( Accessed 10/19/2018.
  • Eunice Kennedy Shriver National Institute of Child Health and Human Development. Phenylketonuria (PKU): Condition Information. ( Accessed 10/19/2018.
  • National Organization for Rare Diseases. Phenylketonuria. ( Accessed 10/19/2018.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy