- Concomitant forbidden medication as described in the Kuvan® Summary of Product Characteristics, namely, inhibitors of dihydrofolate reductase (for example, methotrexate, trimethoprim), medications that are known to affect nitric oxide synthesis (for example, glyceryl trinitrate, isosorbide dinitrate, sodium nitroprusside, molsidomin, phosphodiesterase type 5 inhibitors, and minoxidil), and levodopa, as it may cause increased excitability and irritability