Glycerol phenylbutyrate binds with other substances in the liver and kidneys to help eliminate nitrogen from the body. Excess nitrogen can cause hyperammonemia (HYE-per-AM-moe-NEE-mee-a), a build-up of ammonia in the blood. Ammonia is very toxic when it circulates in blood and tissues and can cause permanent brain damage, coma, or death. Glycerol phenylbutyrate helps prevent a build-up of ammonia in the blood in people with urea cycle disorder. This medicine is used in adults and children who are at least 2 years old. Glycerol phenylbutyrate will not treat hyperammonemia. Glycerol phenylbutyrate is usually given together with a low-protein diet and sometimes dietary supplements. Glycerol phenylbutyrate may also be used for purposes not listed in this medication guide.