A very common (>10% of patients) adverse reactions to isosorbide mononitrate is throbbing headache. The incidence of headache diminishes gradually with time and continued use. At the start of therapy or when the dosage is increased, hypotension and/or lightheadedness in the upright position are commonly observed (ie in 1-10% of patients). These symptoms may be associated with dizziness, drowsiness, reflex tachycardia and a feeling of weakness.