Increased antihypertensive effect w/ other vasodilators; antihypertensives (eg, β-blockers, Ca antagonists, diuretics, ACE inhibitors); TCAs or neuroleptics; alcohol; sapropterin; phosphodiesterase-5 inhibitors (eg, sildenafil, vardenafil). Enhanced vasodilatory effect w/ N-acetylcysteine. Patients previously treated w/ organic nitrates (eg, isosorbide dinitrate, isosorbide-5-mononitrate) may require higher dose of glyceryl trinitrate to achieve desired effect. Raised levels of dihydroergotamine. Weakened effect of heparin.