Health condition
Phenylketonuria in adults
Phenylketonuria (PKU) is detected by newborn screening and managed with a low-phenylalanine diet - but adults need ongoing specialist care, especially women planning pregnancy, and new treatments are changing the landscape.
Why trust this guide
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Editorial team
Written by our editorial team from the UK sources listed on each page.
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Sourced from guidance
Checked against current NHS and UK specialist society guidance at the time of review.
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Current for 2026
Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.
Key facts
The essentials at a glance.
Which gene and test confirm it, who should be tested in the family, and which specialist to see.
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Caused by PAH gene variants; autosomal recessive - newborn heel-prick screening has caught almost all UK cases since 1969
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High phenylalanine in adulthood affects concentration, mood and executive function - 'diet for life' is now standard advice
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Maternal PKU: uncontrolled levels in pregnancy seriously harm the baby, so pre-conception control is essential
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Sapropterin and enzyme substitution therapy (pegvaliase) offer options beyond diet for some adults
Understanding the condition
What it is and what happens next.
What causes it, how it is confirmed, and the testing and specialist routes available in the UK.
What it is and how it presents
Adults with PKU grew up on a strict diet many were told they could relax as teenagers - advice since reversed, because high phenylalanine in adulthood measurably affects attention, processing speed and mood. Many adults have drifted from clinic follow-up entirely. For women, the stakes are higher still: high maternal phenylalanine damages the developing baby's heart and brain, so pregnancy must be planned with levels controlled beforehand.
Testing, surveillance and specialist care
Adult PKU care belongs in a specialist metabolic clinic with dietitians who can modernise the diet, arrange home blood-spot monitoring and assess eligibility for sapropterin or pegvaliase. Adults who lost contact with services years ago can and should be re-referred. NHS clinical genetics services carry some of the longest waits in the health service, often many months for a first appointment, whereas private genetic testing and a specialist review can typically be arranged within around two weeks.
Seeing a specialist
Who to see, and how quickly you can be seen.
The specialist for this condition is usually a metabolic physician (inherited metabolic disease clinic). Your GP can refer you on the NHS, or you can arrange a private consultation directly.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any emergency symptoms.
Frequently asked
Common questions about Phenylketonuria in adults.
Quick, plain-English answers to what people ask us most.
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What is phenylketonuria (PKU) in adults?
PKU is a genetic condition in which the body cannot properly break down phenylalanine, an amino acid found in protein-containing foods, leading to its build-up, which can affect brain function if not managed. Adult PKU refers to lifelong management of this inherited condition.
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What causes PKU?
It's caused by an inherited deficiency of the enzyme phenylalanine hydroxylase, needed to break down phenylalanine. It's passed on when a person inherits a faulty copy of the relevant gene from each parent.
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What are the symptoms of PKU in adults who stop diet management?
Adults who relax strict dietary control may experience difficulties with concentration, mood changes, anxiety, and, over time, potential effects on cognitive function, even though the dramatic effects seen in untreated infants don't reappear in the same way.
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Is PKU hereditary?
Yes, PKU is a classic inherited genetic condition, passed on in an autosomal recessive pattern, meaning both parents must carry a faulty gene copy for a child to be affected.
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Can PKU be treated in adulthood?
Yes, lifelong dietary management restricting phenylalanine intake remains the mainstay of treatment, and newer medications are available for some adults to help manage phenylalanine levels alongside diet.
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Is lifelong dietary management necessary for PKU?
Current guidance recommends lifelong attention to phenylalanine intake for most adults with PKU, as maintaining reasonable control supports better cognitive and mental health outcomes, even though the strictness needed can be less than in early childhood.
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How is PKU monitored in adults?
Regular blood tests to measure phenylalanine levels, along with dietetic review, help guide ongoing management and ensure levels remain within a safe range for long-term wellbeing.
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Does PKU affect pregnancy?
Yes, maintaining very tight control of phenylalanine levels before conception and throughout pregnancy is essential, as high levels can significantly affect the developing baby, so specialist input is important for anyone with PKU planning a pregnancy.
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PKU vs other amino acid disorders — what's the difference?
PKU specifically involves an inability to break down phenylalanine, while other inherited metabolic disorders affect different amino acids or metabolic pathways; each requires its own specific dietary and medical management plan.
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When should I see a specialist, and how quickly can I be seen privately?
Adults with PKU should have ongoing specialist metabolic clinic follow-up; if you're newly transitioning care or seeking a second opinion, private specialist assessment is typically available within about two weeks, faster than many NHS specialist metabolic clinic waits.