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How KUVAN Works

Food and Phe

When you eat foods with protein, your body breaks the protein down into several different building blocks called amino acids. One of these is phenylalanine, or Phe.

Working at the Source of the Problem

In the body, the PAH enzyme converts Phe into tyrosine in the presence of tetrahydrobiopterin (BH4). BH4 is a natural substance found in the body that helps the PAH enzyme reduce Phe to safe levels in the blood.

The active ingredient in KUVAN® (sapropterin dihydrochloride) Tablets or Powder for Oral Solution is a pharmaceutical version of BH4. It works in the same way as your body’s BH4. KUVAN adds more BH4 and stimulates the PAH enzyme to process Phe in people with PKU.

The PAH enzyme begins to convert Phe to tyrosine, and helps to keep your blood Phe levels low.

KUVAN has been prescribed along with a low-Phe diet to lower blood Phe levels in many people with PKU of all ages and across all ranges of symptoms.

KUVAN: A Proven Way to Treat PKU

Food and Phe: When you eat foods with protein, your body breaks the protein down into several different building blocks called amino acids. One of these is phenylalanine, or Phe.

Key Terms

Phe—short for phenylalanine, this is an amino acid that comes from protein and is toxic to the brain at high levels

Enzyme—something your body makes that changes a chemical from one thing to another

PAH—short for phenylalanine hydroxylase, an enzyme that works with BH4 to break down Phe and is either missing or not working properly in people with PKU

BH4—short for tetrahydrobiopterin, a helper (also known as a co-factor) of the enzyme PAH

T—short for tyrosine, an amino acid that works as a building block for several important brain chemicals

People without PKU

PAH enzyme

People without PKU can break Phe down into tyrosine with an enzyme called PAH and its helper, BH4.

People with PKU

PAH enzyme

People who have PKU were born with PAH that’s missing or not working properly, which makes it hard to break down Phe. This causes a buildup of Phe that can have toxic effects in the body and brain.

People with high Phe levels may experience anxiety, depression, confusion, trouble focusing, and other symptoms that can affect the way they think, feel, and act.

People with PKU taking KUVAN

KUVAN helping
the PAH enzyme

KUVAN acts like the body’s own BH4 to help break down Phe in people with PAH that is missing or not working properly.

In combination with a low-Phe diet, KUVAN can help reduce blood Phe levels in those who respond.

Clinical experts recommend a trial of KUVAN for all patients with PKU.*

*Except for those with 2 null mutations in trans.


The most common side effects reported when using KUVAN are headache,
runny nose and nasal congestion, sore throat, diarrhea, vomiting, and cough.

What is KUVAN?

KUVAN® (sapropterin dihydrochloride) Tablets for Oral Use and Powder for Oral Solution are prescription medicines used to lower blood Phe levels in adults and children over one month of age with a certain type of Phenylketonuria (PKU). KUVAN is to be used along with a Phe-restricted diet.

What is the most important information I should know about KUVAN?
KUVAN can cause serious side effects,

  • Severe allergic reactions including anaphylaxis. Stop taking KUVAN and get medical help right away if you develop any of these signs or symptoms of a severe allergic reaction:
    • Wheezing, coughing, or trouble breathing
    • Feeling flushed, nauseous, lightheaded or you faint
    • Rash
  • Inflammation of the lining of the esophagus or stomach (esophagitis and gastritis). Your doctor will monitor you for symptoms of inflammation in your upper gastrointestinal tract, including your stomach and esophagus. If untreated that inflammation can lead to serious side effects including narrowing of the esophagus, ulcers, and bleeding. Call your doctor right away if you have any of these signs or symptoms:
    • Pain in the upper abdomen (stomach area), esophagus, or throat
    • Nausea, trouble swallowing, loss of appetite, or vomiting
    • Blood in your vomit or stool
    • Black, tarry stools
  • Phe levels that are too low. Patients have experienced low blood Phe during treatment with KUVAN. Low blood phe is more common in children under the age of 7 who take high doses of KUVAN each day.
  • Too much or constant activity (hyperactivity) can happen with KUVAN. Tell your doctor if you have any signs of hyperactivity, including:
    • Fidgeting or moving around too much
    • Talking too much

What should I tell my doctor before I take KUVAN?
Before you take KUVAN, tell your doctor about all your medical conditions, including if you:

  • Are allergic to sapropterin dihydrochloride or any of the ingredients in KUVAN
  • Have poor nutrition or have a loss of appetite
  • Are pregnant or plan to become pregnant
  • Are breastfeeding or plan to breastfeed. It is not known if KUVAN passes into your breast milk. Talk to your doctor about the best way to feed your baby if you take KUVAN.

Tell your doctor about all the medicines you take, including prescription and over-the-counter medicines, vitamins, and herbal and dietary supplements. KUVAN and other medicines may interact with each other. Especially tell your doctor if you take:

  • A medicine that contains levodopa
  • An antifolate medicine such as methotrexate, valproic acid, phenobarbital, trimethoprim
  • Sildenafil (Revatio, Viagra), tadalafil (Adcirca, Cialis), vardenafil (Staxyn, Levitra)

Tell your doctor if you are not sure if your medicine is one that is listed above. Know the medicines you take. Keep a list of them to show your doctor and pharmacist when you get a new medicine.

How should I take KUVAN?
See the detailed “Instructions for Use” that comes with KUVAN for information about the correct way to dissolve and take a dose of KUVAN tablets or KUVAN powder for oral solution.

  • KUVAN does not work for everyone. It is not possible to know if KUVAN will work for you until you start taking KUVAN. Your doctor will check your blood Phe levels when you start taking KUVAN to see if the medicine is working.
  • Take KUVAN exactly as your doctor tells you
  • You can swallow KUVAN tablets whole or dissolve the tablets in water or apple juice
  • KUVAN powder for oral solution should be dissolved in water or apple juice
  • KUVAN should be taken with a PKU-specific diet. Any change you make to your diet may affect your blood Phe level. Follow your doctor’s instructions carefully and do not make any changes to your dietary Phe intake without first talking with your doctor. Even if you take KUVAN, if your blood Phe levels are not well controlled, you can develop severe neurologic problems.
  • Your doctor should continue to monitor your blood Phe levels often during your treatment with KUVAN to make sure that your blood Phe levels are not too high or too low and frequent monitoring for pediatric patients is recommended. Both high and low Phe can result in serious problems. Active management of dietary Phe intake while taking KUVAN is required.
  • If you have a fever, or if you are sick, your blood Phe level may go up. Tell your doctor as soon as possible so he or she can change your dose of KUVAN to help keep your blood Phe levels in the desired range.
  • If you forget to take your dose of KUVAN, take it as soon as you remember that day. Do not take 2 doses in a day.
  • If you take too much KUVAN, call your doctor for advice

The most common side effects of KUVAN are: headache, runny nose and nasal congestion, sore throat, diarrhea, vomiting, and cough.

These are not all the possible side effects of KUVAN. For more information, ask your doctor or pharmacist. Call your doctor for medical advice about side effects.

You may report side effects to the FDA at 1-800-FDA-1088 or You may also report side effects to BioMarin Pharmaceutical Inc. at 1-866-906-6100.

Please read the full Prescribing Information by clicking here.