Beta blocker: A class of drugs that block the effect of beta-adrenergic substances such as adrenaline (epinephrine), that play a key role in the sympathetic portion of the involuntary nervous system. By blocking the action of the sympathetic nervous system on the heart, they slow the heartbeat and relieve stress on the heart. Beta blockers are used to treat abnormal heart rhythms, specifically to prevent abnormally fast heart rates (tachycardias) or irregular heart rhythms, such as premature ventricular beats. Because beta blockers reduce the demand of the heart muscle for oxygen, they can be useful in treating angina. They have also become important drugs in improving survival after a heart attack. Due to their effect on blood vessels, beta blockers can lower the blood pressure and are of value in the treatment of hypertension. Other uses include the prevention of migraine headaches and the treatment of familial or hereditary essential tremors. Beta blockers reduce pressure within the eye and they are therefore used to lessen the risk of damage to the optic nerve and loss of vision in patients with glaucoma. Beta blockers include acebutolol (Sectral), atenolol (Tenormin), bisoprolol (Zebeta), metoprol (brand names: Lopressor, Lopressor LA, Toprol XL), nadolol (Corgard), and timolol (Blocadren). Topical beta blockers for the eye include timolol ophthalmic solution (Timoptic) and betaxolol hydrochloride (Betoptic).