Kuvan, produced by Biomarin Pharmaceutical Inc., is a drug that, in some people with PKU, can enhance enzyme activity and lower blood phe levels. Kuvan is a form of the cofactor for the phenylalanine hydroxylase enzyme, called tetrahydrobiopterin or BH4. When given in sufficiently large doses, it can cause whatever residual enzyme activity exists to work harder and thereby reduce blood phe levels. It is most likely to be effective in those with significant residual enzyme activity, such as those with Ã¢â‚¬Å“variantÃ¢â‚¬Â PKU or mild hyperphenylalaninemia. It is not possible to know in advance whether an individual with PKU will be responsive to the drug, so it must be tried for a length of time to determine if it will be effective. If it is effective, then a dosage requirement needs to be determined by the doctor. In rare cases, someone on the diet has been able to discontinue the diet entirely; but more typically, use of Kuvan has resulted in greater tolerance for dietary phenylalanine while the diet is still maintained.