Beta-blockers work by blocking beta receptors that are found in various parts of the body. Blocking beta receptors prevents the action of two chemicals, called noradrenaline and adrenaline, which are produced naturally by the body. These are often referred to as the ‘fight or flight’ chemicals, because they are responsible for the body’s reaction to stressful situations.In addition to these heart related uses, propranolol can also be used to control symptoms that are associated with anxiety or an overactive thyroid gland (thyrotoxicosis), such as a racing pulse, sweating and tremor. This is because it blocks the effects of adrenaline. Propranolol is also used to prevent migraine, although it is not fully understood how propranolol works in this area.