Has anyone had any issues with a beta blocker such as Atenolol, meant to control high blood pressure? Last week I was on a mountain bike ride, smooth trail, but with constant rising grade. I was working pretty hard, began to be dizzy, nauseous, major headache. I stopped and nearly collapsed. ( I had had plenty of water and food) Luckily I was with a friend (I often ride alone) who convinced me after a half hour of not feeling any better, that she should ride out and call the medics. To make a long story short, I ended up in the hosp emerg room, and the attending doc felt it was the extra effort hitting up against the meds. My regular doc, whom I saw the next day, didn't agree but had no better explanation than I may have had a mild infection due to a tiny increase in white cell count (not confirmed in a 2d test).