Sapropterin dihydrochloride is primarily used in the treatment of phenylketonuria (PKU), a genetic disorder where the body cannot properly metabolize phenylalanine. By helping to reduce phenylalanine levels in the blood, it aids in managing the condition and preventing associated complications such as intellectual disabilities and neurological issues. It is often prescribed as part of a specialized diet to enhance its effectiveness. Additionally, it may be used in certain cases of tetrahydrobiopterin (BH4) deficiency, supporting the body's ability to process amino acids and neurotransmitters. Regular monitoring of blood phenylalanine levels is essential during treatment to ensure optimal dosing and efficacy.