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Phenylketonuria (PKU)

Phenylketonuria (PKU) is a rare but potentially serious inherited disorder.

PKU causes the amino acid phenylalanine to build up in the blood and brain. This can lead to brain damage.

About 1 in 4,500 babies born in Ireland has PKU. With early diagnosis and treatment most go on to live healthy lives.

Check if it's PKU

Babies can have heel prick screening at around 5 days old. This is to check if they have PKU or other conditions.

A midwife or nurse will take a few drops of blood from your baby's heel using a special device.

If PKU is confirmed, treatment will be given straight away. This is to reduce the risk of complications. Treatment includes a special diet and regular blood tests.

Symptoms of PKU

PKU does not usually cause any symptoms if treatment is started early.

Without treatment, PKU can damage the brain and nervous system. This can lead to learning disabilities.

Other long-term symptoms include:

  • behavioural difficulties such as temper tantrums and self-harm
  • lighter skin, hair and eyes
  • eczema
  • recurrent vomiting
  • jerking movements in arms and legs
  • tremors
  • epilepsy
  • musty smell on the breath, skin and urine

Treating PKU

Treatment includes a special diet and regular blood tests.


The main treatment is a diet that avoids high-protein foods such as meat, eggs and dairy products. It also controls the intake of foods such as potatoes and cereals.

People with PKU must take amino acid supplements. This is to make sure they're getting all the nutrients needed for normal growth and good health.

There are low-protein versions of popular products (such as flour, rice and pasta) for people with PKU.

If your baby has high phenylalanine levels they will start on a low-protein diet and amino acid supplements straight away.

A dietitian will make a detailed plan for your child. This can be changed as they grow and their needs change.


People with PKU must avoid food products with aspartame as it is converted into phenylalanine in the body.

Aspartame is a sweetener found in:

  • artificial sweeteners often used in tea and coffee
  • diet versions of fizzy drinks
  • chewing gum
  • squashes and cordials
  • some alcopops
  • some children's cold and flu medicine

All food products and medicine that contain aspartame should be clearly labelled.

Regular blood tests

A child with PKU needs regular blood tests. This is to measure levels of phenylalanine and to see if they're responding to treatment.

The hospital will only need a drop of blood. With training you may be able to collect this at home and send it to the hospital by post.

It's recommended that children:

  • 6 months or younger have blood tests once a week
  • 6 months to 4 years have blood tests once every 2 weeks
  • 4 years and up have blood tests once a month

Someone with PKU will usually need to have regular blood tests throughout their life.


In some cases, doctors may prescribe a medicine called sapropterin. This can help to break down phenylalanine so it will not build up in the blood.

How a child inherits PKU

PKU is caused by a gene passed on by both parents. They are usually carriers and do not have symptoms themselves.

If you're a carrier of the altered gene and you have a baby with someone who's also a carrier, your baby has a:

  • 25% chance of having PKU
  • 50% chance of being a carrier of PKU
  • 25% chance of receiving a pair of normal genes

Adults with PKU

Many adults with PKU function best on a low-protein diet. The current advice for most is to remain on a low-protein diet for life.

There is not any evidence that high phenylalanine levels cause permanent brain damage in adults. But they may lose concentration or have a slower reaction time.

This can usually be reversed by going back on to a stricter diet. This will bring phenylalanine levels down again.

PKU and pregnancy

Women with PKU have to take special care during pregnancy. This is because high levels of phenylalanine can hurt an unborn baby.

If phenylalanine levels are controlled problems can be avoided. Women with PKU are able to have normal, healthy babies.

It's recommended that you plan your pregnancy carefully. Follow a strict diet and check your blood twice a week before trying to become pregnant. You should try to conceive when phenylalanine levels are within the target range for pregnancy.

Once your baby is born the strict diet can be relaxed and you can breastfeed as normal.

Contact your doctor or dietitian as soon as possible if you get pregnant when your phenylalanine levels are not controlled. If you can bring them down within the first few weeks of your pregnancy, the risk of damage to the baby should be small.

Page last reviewed: 27 October 2021
Next review due: 27 October 2024

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This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 8.