A 7.5-year-old girl who was treated with phenobarbital (PHB) for epilepsy was admitted with decreased levels of consciousness. She had been known to have high PHB levels of unknown cause, without symptoms. Her PHB levels were very high, as expected, but primidone levels were also detected although she and her parents denied history of primidone administration. We wished to rule out intentional unprescribed use of primidone. Our retrospective review showed 3 other children with high PHB concentrations where primidone was also detected when PHB levels were over 130 μmol/L. Complementary studies confirmed that high-dose PHB can convert to its prodrug primidone, which has not been reported previously.