These prescription-only medicines interrupt the transmission of impulses (messages) across nerve cells (neurons). In the HEART these are called beta1 (cardioselective) receptors. Another type – beta2 (non-cardioselective) receptors – are sited in the airways, blood vessels, and organs such as the eye, liver and pancreas. Cardioselective beta blockers act primarily on beta1 receptors, whereas non-cardioselective drugs act on both varieties. (The neurotransmissions interrupted at the beta-receptor sites through the body by the beta blockers are initiated in the ADRENAL GLAND: this is why these drugs are sometimes described as beta-adrenergic-blocking agents.

They work by blocking the stimulation of beta adrenergic receptors by the neurotransmitters adrenaline and noradrenaline, which are produced at the nerve endings of that part of the SYMPATHETIC NERVOUS SYSTEM which facilitates the body's reaction to anxiety, stress and exercise – the ‘fight and flight’ response.

Beta1 blockers reduce the frequency and force of the heartbeat; beta2 blockers prevent vasodilation (increase in the diameter of blood vessels), thus influencing blood pressure. They can reduce to normal an abnormally fast heart rate so the power of the heart can be controlled: this reduces the oxygen requirements of the heart with an advantageous knock-on effect on the respiratory system. These are valuable therapeutic effects in patients with ANGINA or who have had a myocardial infarction (heart attack – see HEART, DISEASES OF), or who suffer from HYPERTENSION. Beta2 blockers reduce tremors in muscles elsewhere in the body which are a feature of anxiety or the result of THYROTOXICOSIS (an overactive thyroid gland). Non-cardioselective blockers also reduce the abnormal pressure caused by the increase in the fluid in the eyeball that characterises GLAUCOMA.

Many beta-blocking drugs are now available; minor therapeutic differences between them may influence the choice of a drug for a particular patient. Among the common drugs are:

These drugs have various side-effects and should be prescribed and monitored with care. In particular, people who suffer from asthma, bronchitis or other respiratory problems may develop breathing difficulties. Long-term treatment with beta blockers should not be suddenly stopped, as this may precipitate a severe recurrence of the patient's symptoms – including, possibly, a sharp rise in blood pressure. Gradual withdrawal of medication should mitigate untoward effects.