DL-Phenylalanine, pure pharmaceutical USP grade powder. L-Phenylalanine is the natural form of Phenylalanine and is important for building proteins and maintaining a healthy mood. D-Phenylalanine is the laboratory version and is important as a pain-relief substance. When taken together, DL-Phenylalanine has great benefits in improving focus and attention. Phenylalanine supplements may interact with other medicines and should not be taken by people suffering from phenylketonuria.