What is sepiapterin?
Sepiapterin is an oral medicine for phenylketonuria (PKU). It is taken once a day. It enters cells where it helps the body's PAH enzyme convert phenylalanine (Phe) into tyrosine more efficiently. The result is lower blood Phe levels in many people with PKU - including some who did not respond to sapropterin.
The powder is mixed with water, apple juice, or soft food (such as apple sauce) and swallowed within 30 minutes after a meal that contains fat.
Could it be for me?
The international consensus suggests offering a sepiapterin trial to anyone with PKU whose blood Phe has ever been above 360 µmol/L. This page is not a diagnostic tool; the candidate question list below is for discussion with your team.
Who can be considered:
- people with mild to severe PKU;
- those who started treatment late in life;
- people who did not respond to sapropterin;
- people who are or are not currently on a strict Phe-restricted diet;
- people taking other PKU treatments (with team discussion).
Approval ages by region:
- Europe, Australia, Switzerland: all ages.
- United States, Canada: from 1 month of age, alongside a Phe-restricted diet.
This guide does not cover infants under 2 years of age. A separate document for that group is in preparation.
Listen or watch
Two short companion formats summarize the same content.
Step 1: Baseline and preparation
Before starting a sepiapterin trial:
- your team explains the process, expected outcomes, and any insurance considerations;
- a baseline assessment is done: blood Phe (ideally three samples), diet history, body measurements, prior treatments;
- diet should be stable for at least one week before starting the test;
- blood Phe levels should ideally be at or above 360 µmol/L at baseline so the response is clear.
Step 2: The 2-week response test
The first 14 days check whether sepiapterin is reducing blood Phe.
- Sepiapterin is taken every day as prescribed.
- At least 3 blood Phe tests are recommended during the trial (for example, day 5, 10, and 14).
- Blood is drawn at the same time of day, under the same conditions, each time.
- A short-term response is usually a 30 percent or greater drop in blood Phe.
- The trial can be extended by another 2 weeks if you become unwell or results are unclear.
Step 3: Long-term care (3 to 6 months)
If short-term response is confirmed, a long-term trial follows.
Goals: lower Phe within target range, eat more natural protein, or both.
Monitoring during this phase:
- blood Phe checks continue and become more frequent when diet changes;
- wait for two stable Phe results before increasing natural protein;
- reassess metabolic status, growth, micronutrients, eating patterns over time.
If natural protein is increased and Phe climbs above target, the increase must be paused or reversed. Metabolic control comes first.
If Phe drops below 30 µmol/L, increase dietary Phe immediately - confirm full protein dose first. The team may consider reducing the sepiapterin dose if needed.
Take it with food
To improve absorption, take sepiapterin within 30 minutes of a meal that contains fat. About 8 to 10 grams of fat in the meal is suggested for children over 2 years. Optimal amounts have not yet been formally studied.
Low-protein fat sources:
- Avocado
- Coconut
- Olives
- Cooking oil
- Butter
- Modified low-protein milks
- Plant creams
- Low-protein cheese
- Low-protein pancakes/waffles
- Vegan mayonnaise
High-protein fat sources:
- Full-fat cow's milk
- Full-fat yogurt
- Eggs
- Cheese
- Soft cheese
- Nuts
- Seeds
- Nut butter
- Tofu
- Fatty fish (tuna, salmon)
- Butter croissants
- Nut bars
Bringing back natural protein
If long-term response is confirmed, the dietitian guides you through four stages. Pace and choices are individual - this is a map, not a fixed schedule.
- 1 Replace some special low-protein foods. Gradually swap a few low-protein or low-Phe products for regular versions, where blood Phe allows.
- 2 Add measured natural protein. Protein-containing vegetables, whole-grain cereals, pulses, whole eggs, pre-portioned cheese, yogurt pots, measured volumes of milk, nuts and seeds.
- 3 Expand variety. Add meat or fish products, soy-based products, tofu. Start with lower-protein versions before higher-protein ones.
- 4 Reduce protein substitute. As natural protein intake grows, your dietitian reduces the substitute dose proportionally. Some people stop completely; others keep a small dose for safety.
Throughout these stages, food diaries and regular blood Phe checks guide every decision. Watch for new food sensitivities when introducing common allergens (such as nuts or eggs) - introduce one new food at a time, in small amounts.
Healthy choices and nutrient checks
More natural protein does not mean more processed foods. Choose nutritious options: fruit, vegetables, whole grains, lean protein.
When the protein substitute is reduced, vitamins and minerals are monitored: vitamin B12, iron, vitamin D, calcium, iodine, zinc. Supplements may be needed.
Side effects: what to expect
Sepiapterin is generally well tolerated. The most common reported effects in studies were:
- upper respiratory infections;
- headache;
- diarrhea;
- abdominal pain.
Stools may change color while you are on treatment.
The mixed liquid tastes sweet (especially in apple juice) and is a little thick. It can take time to get used to.
Some people notice mild, temporary tooth color change. This is resolved by normal brushing and rinsing the mouth with water after each dose.
Call your team if: a side effect lasts more than a few days or affects daily life; you have signs of an allergic reaction (rash, swelling, trouble breathing - seek urgent care); your blood Phe drops below 30 µmol/L on repeated tests.
Sick-day plan
During fever or illness, blood Phe levels often rise. Sepiapterin-specific data on illness are still limited, so the consensus is to maintain the same vigilance you would use during sapropterin treatment.
Practical steps:
- follow your clinic's existing sick-day plan;
- keep fluids and energy intake up;
- if you have reduced or stopped your protein substitute, your team may ask you to use a small dose temporarily during illness;
- contact your clinic if symptoms persist or you cannot keep food and fluids down.
If sepiapterin is stopped
If sepiapterin is stopped and no other Phe-lowering medicine is started, blood Phe will rise again. Your team will help you return to your previous Phe-restricted diet right away.
Reasons treatment may be paused or stopped: insufficient effect, side effects, difficulty taking the medicine, pregnancy, breastfeeding, or planning a pregnancy.
Pregnancy considerations
If you are pregnant, breastfeeding, or planning a pregnancy, talk with your team in advance about whether sepiapterin should continue. Strict Phe control during pregnancy is essential to protect the baby.
Future updates
This guidance is reviewed every 5 years as new evidence emerges. Localized versions of this text (French, German, Spanish) are in preparation.
Sources
MacDonald A, Ahring K, Bledsoe A, et al. Dietetic guidance for nutritional management of people with phenylketonuria receiving sepiapterin. Mol Genet Metab. 2026;147:109705. doi:10.1016/j.ymgme.2025.109705