Which of the following is a common adverse reaction associated with the use of calcium channel blockers?

Calcium channel blockers are primarily used to treat conditions like hypertension and angina by inhibiting the influx of calcium ions into cardiac and smooth muscle cells. This action can lead to several physiological effects, including vasodilation and decreased heart rate. Among the common adverse reactions, bradycardia, which is defined as a slower-than-normal heart rate, is particularly notable. This occurs because calcium influx is critical for cardiac conduction and contractility; by blocking this influx, calcium channel blockers can cause a reduction in the heart's conduction velocity and a decrease in heart rate.

In contrast, while headache, nausea, and weight gain can occur with various medications, they are not typical or primary adverse reactions associated with calcium channel blockers. Headaches may result from vasodilation and are seen in some patients, but bradycardia is a more direct and notable outcome of calcium channel blockade. Thus, bradycardia stands out as a key adverse effect that healthcare providers need to monitor for when patients are prescribed calcium channel blockers.