Commonly used to treat hypertension, beta blockers have a big drawback: they can reduce the heart’s capacity for exercise and therefore affect your exercise tolerance. In addition, if medication restricts the heart’s function during exercise, then there is an increased risk of loss of consciousness, which could prove fatal underwater.

Because of this effect on divers, doctors often recommend a stress test. According to Dr. Alfred Bové (Bové and Davis’ Diving Medicine,4th Ed.), divers who use beta blockers and who can achieve a strenuous level of exercise withoutsevere fatigue may be cleared for diving. Bové also mentions that although diving does not usually represent the maximum workload on the heart, divers taking beta blockers should avoid extreme exercise because their maximum capacity for exercise may be reduced.