I'm on a fairly high dose of one for migraine prevention. I didn't feel any different at all. I frequently work in the cardiac care unit at a hospital---it is pretty rare that I hear of a side effect of a beta blocker. The most common things I do hear is occassional nausea, diarrhea, insomnia, and fatigue. One of the more serious side effects is low blood pressure. Most of the minor side effects clear up in a few days. If it is anything more than a minor side effect, I would call your doc.