Used to treat phenylketonuria (PKU), a genetic disorder that affects the body's ability to process phenylalanine. It helps lower blood levels of phenylalanine in patients who are deficient in the enzyme phenylalanine hydroxylase. By improving enzyme activity, it allows for increased tolerance to dietary phenylalanine, reducing the need for strict protein restriction. This leads to better nutritional outcomes and improved quality of life. Most effective in patients who respond to the treatment, as determined by a diagnostic trial. Commonly used in both pediatric and adult populations under medical supervision.