Beta-blocker drug molecule. Computer model of a molecule of the beta-blocker drug atenolol. Atoms are represented as spheres and are colour-coded: carbon (purple), hydrogen (white), oxygen (yellow) and nitrogen (pink). Beta-blockers are a class of drugs used to treat several cardiovascular diseases, including coronary heart failure, arrhythmia (irregular heart beat) and hypertension (high blood pressure). They work by blocking the receptors for the hormones adrenaline and noradrenaline in the heart muscle and blood vessels. Atenolol reduces the strength of the heart's contractions, reduces the heart's oxygen requirements and increases blood vessel diameter, which decreases blood pressure.