Beta-2-Agonists are dilators which cause dilation (widening) of vessels by relaxing the smooth muscle surrounding them.

Medical Uses of Beta-2-Agonists

Inhaled forms of Beta-2-agonists (including salmeterol, salbutamol, formoterol, and terbutaline) are used for the treatment of Asthma and exercise-induced bronchospasm (EIB). 
Injected forms are most commonly used in the prevention of premature labour, as they act to relax the smooth muscle of the uterus and so inhibit contractions