What are potential serious adverse effects of beta-blockers?

Rebound hypertension, if not tapered
Decreased cardiac output

Beta-blockers must be tapered when discontinuing, otherwise the patient will experience rebound hypertension. The mechanism is thought to be increased sensitivity and up-regulation of the beta-receptors, since they have been blocked by the beta-blocker. Thus, when the beta-blocker is discontinued, these receptors are now free to be extra sensitive to the effects of norepinephrine/epinephrine and thus hypertension is observed. Keep in mind that beta-blockers decrease cardiac output. They decrease the heart rate and contractility. This allows the heart to decrease its workload and spend more time in diastole, which increases perfusion. However, this therapeutic mechanism can become dangerous, if the heart rate is decreased excessively, such as in cardiogenic shock. Hepatotoxicity and hypermagnesemia are not particularly associated with beta-blockers.