If phenylketonuria (PKU) is not treated, what problems occur?

Children and adults who do not receive treatment for PKU may develop a variety of symptoms.

  • Children with PKU who are not treated may develop symptoms including behavioral problems, seizures, and severe intellectual and developmental disabilities (IDDs).1
  • Adults with PKU who do not follow a special diet may develop unstable moods2 and take longer to process information.3 Adults with high phenylalanine levels who go back on a PKU diet may be able to improve their mental functioning and slow down any damage to their central nervous systems.4
  • Pregnant people with PKU who do not strictly follow a low-phenylalanine diet may give birth to a child with serious problems, including IDDs, a head that is too small (microcephaly), heart defects, and low birth weight.5 Read more about maternal PKU. Women with PKU and uncontrolled phenylalanine levels also have an increased risk of pregnancy loss.1


  1. MedlinePlus. (2023). Phenylketonuria. Retrieved December 21, 2023, from https://medlineplus.gov/ency/article/001166.htm
  2. Bélanger-Quintana, A., Burlina, A., Harding, C. O., & Muntau, A. C. (2011). Up to date knowledge on different treatment strategies for phenylketonuria. Molecular Genetics and Metabolism, 104 Suppl(0), S19–S25.
  3. NICHD. (2000, updated 2006). Report of the NIH Consensus Development Conference on Phenylketonuria (PKU): Screening and management. Retrieved May 15, 2012, from https://www.nichd.nih.gov/publications/pubs/pku/index
  4. Dolan, B. E., Koch, R., Bekins, C., & Schuett, V. (2000). Diet intervention guidelines for adults with untreated PKU. National PKU News. Retrieved June 22, 2012, from https://pkunews.org/diet-intervention-guidelines-for-adults-with-untreated-pku/ external link
  5. National Human Genome Research Institute. (2010). Learning about phenylketonuria (PKU). Retrieved May 15, 2012, from https://www.genome.gov/Genetic-Disorders/Phenylketonuria#al-1
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