Verapamil is a type of calcium channel blocker I take for my AV Nodal Reentry Tachycardia. It keeps my heart rhythm steady even in periods of stress or during exercise. One downside is that it makes me exceedingly intolerant of heat. I get hot easily, and heat exhausts me quickly. I find that I have tachycardia or atrial flutter more often when hot. My cardiologist, however, says this is not because of the medicine. Whatever.