Atenolol is a drug belonging to the group of beta blockers, a class of drugs used primarily in cardiovascular diseases. Introduced in 1976, atenolol was developed as a replacement for propranolol in the treatment of hypertension. Propranolol is known to readily cross the blood-brain barrier (BBB) and can pass into the brain, causing side-effects such as depression and nightmares; atenolol was specifically developed to be unable to pass through the blood-brain barrier in order to prevent this effect. Whilst atenolol, the most widely used beta-blocker in the United Kingdom, was once the first-line treatment for hypertension, the role for beta-blockers in hypertension was downgraded in June 2006 in the United Kingdom to fourth-line as they perform less well than other drugs, particularly in the elderly, and there is increasing evidence that the most frequently used beta-blockers at usual doses carries an unacceptable risk of provoking type 2 diabetes.