Overview
The body can't break down phenylalanine, found in protein.
Newborn screening finds it early.
Symptoms
Common signs:
- Found on newborn screen
Less common
- Musty smell
When to get medical care
Causes
Genes.
Does it spread or run in families?
- It isn't contagious. You can't catch it from someone else.
Diagnosis
A clinician asks about your symptoms and may order tests.
The exam
A physical exam.
Tests
Tests, if needed:
- Blood test:
Treatment
Treatment depends on the cause and how severe it is.
Self-care
- Follow a low-phenylalanine diet
- Avoid aspartame
Prevention
To lower your risk or prevent problems:
- Newborn screening
Which doctor treats this
- Start with your primary care provider (family doctor or internist).
- They may send you to a medical geneticist (genetics doctor).
Living with it
- Keep a simple diary of symptoms, triggers, and medicines to bring to visits.
- Bring a list of your medicines and questions to each visit.
- Stay active in ways that feel good to you.
- Get enough sleep and manage stress.
- Long-term illness can affect mood. Tell your doctor if you feel down or anxious.
Support groups and helplines
- Genetic and Rare Diseases Information Center (GARD): Free NIH information on rare diseases, including where to find experts and patient groups.
- National Organization for Rare Disorders (NORD): Patient groups, help finding care, and aid programs for rare diseases.
Back to school, work, or sports
- Most people can keep working or going to school.
- Ask about changes that help, like extra breaks or a flexible schedule. Workers may be covered by the ADA. Students may get a 504 plan.
Children, pregnancy, and older adults
- Pregnancy: Strict diet control protects the baby.
- Women with PKU: Strict diet before pregnancy.
Pregnancy and breastfeeding
- If you're pregnant or planning to be, tell your prenatal provider about this condition.
- Don't start or stop any medicine on your own. Some are safe in pregnancy and some need to be changed.
- Ask before taking any pain reliever. The FDA advises avoiding ibuprofen and naproxen after 20 weeks unless your doctor says to.
- Breastfeeding: Most medicines are fine, but ask. The NIH LactMed database lists what's known about each one.
For caregivers
- Go to visits when you can and take notes.
- Keep a list of medicines, allergies, and doctors in one place.
- Learn the warning signs that need fast care.
- Take breaks and ask for help. Caregivers need care too.
Outlook
Lifelong diet keeps the brain healthy.
Getting ready for your visit
Bring a list of your symptoms and medicines.
What to bring
- A list of your medicines and supplements
- Notes on when symptoms started and what makes them better or worse
Questions to ask
- What's causing this?
- What treatment do you recommend?
Research studies
Research studies test new ways to find and treat health problems. Some are looking for volunteers. Talk with your doctor before joining one.
Search ClinicalTrials.gov for studies on Phenylketonuria (PKU)
Sources
Last updated 2026-11-12.