Aspirin, when given to febrile children under 12, can lead to Reye's syndrom, a very rare but potentially fatal condition. The problems are mainly neuronal (convulsions, hallucinations, irrational behaviour, lethargy) but also affect the liver (swelling, hyperammonaemia, hypoglycaemia). The cause is unclear, but may be the manifestation of an underlying disorder of fatty acid oxydation.