Atrial fibrillation is an irregular heartbeat that can prevent the heart from pumping enough blood, which can lead to fainting, difficulty breathing, and other symptoms. High blood pressure is one of the risk factors for atrial fibrillation. Drugs that lower blood pressure reduce the risk for atrial fibrillation. Some drugs for lowering blood pressure may also reduce the risk for atrial fibrillation by other mechanisms, and these drugs should prevent more atrial fibrillation than drugs that only lower blood pressure. Clinical trials that randomly assign persons with high blood pressure to a placebo or an active drug and record the frequency of atrial fibrillation have produced conflicting results. Older trials found no difference in the frequency of atrial fibrillation with different drugs, but more recent trials have found that some drugs prevent atrial fibrillation more effectively than others.