Due to improved diagnostics and care there is an increasing number of adults with inherited metabolic diseases. The best-known example is phenylketonuria. Treatment consists of a disease-specific diet, for example protein restriction supplemented with essential amino acids. However, like prescription drugs, diet preparations can have side effects. This implies that a description of the indications and contra-indications, an assessment of the efficacy and a definition of the desired duration of treatment are required. Mistakes in the delivery of these disease-specific diet preparations by the pharmacy can have severe consequences, as illustrated by three case reports.