Any of a class of drugs that block impulses
that stimulate certain nerve endings (beta
receptors) serving the heart muscles. 
This reduces the heart rate and the
force of contraction, which in turn reduces
the amount of oxygen (and therefore the blood
supply) required by the heart. Beta-blockers
are banned from use in competitive sports.
They may be useful in the treatment of
angina, arrhythmia, and raised blood
pressure, and following myocardial
infarctions. They must be withdrawn from use
gradually.