BMJ Group Medical Reference
Taking a drug called a beta-blocker can help to slow down your heartbeat.  A big review of good-quality studies (randomised clinical trials) of people with short-term and long-term atrial fibrillation found that beta-blockers helped slow down the hearts of 6 in 10 people, even when they were exercising. 
Beta-blockers include drugs such as atenolol (Atenix, Tenormin), metoprolol (Lopressor), propranolol (Angilol, Bedranol, Beta Prograne, Inderal, Lopranol, Syprol), sotalol (Beta-Cardone, Sotacor), and timolol (Betim). You can have injections in hospital to treat a short-term attack, or you can take tablets at home if your atrial fibrillation lasts a long time. 
Beta-blockers can make you feel light-headed, dizzy, or tired.
If you have asthma, you shouldn't take a beta-blocker, because it can make your breathing worse. If you have diabetes, beta-blockers may prevent your body from recognising that your blood sugar is too low.
randomised controlled trials
Randomised controlled trials are medical studies designed to test whether a treatment works. Patients are split into groups. One group is given the treatment being tested (for example, an antidepressant drug) while another group (called the comparison or control group) is given an alternative treatment. This could be a different type of drug or a dummy treatment (a placebo). Researchers then compare the effects of the different treatments.
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