People who experience certain types of irregular heart rhythms, high blood pressure, or chest pain may benefit from Calan^® (verapamil hydrochloride). This prescription drug works to relax the blood vessels and improves blood flow. Calan is available as a tablet and is usually taken three or four times daily. Some of the common side effects seen with this drug include headaches, nausea, and dizziness. Seek medical attention immediately if you happen to overdose on Calan. Symptoms of a Calan overdose can include decreased mental function, fluid in the lungs, and a slow heart rate.