People with asthma develop intermittent contraction of the muscle in the walls of the tubes (airways, or bronchi) that bring air to the lungs. Breathing becomes difficult when the airways contract. Some people with asthma have infrequent symptoms, others have severe symptoms on most days, and most are somewhere in between. Triggers of asthma symptoms include smoke, infection, allergic reactions, cold weather, and exercise. In some people, the triggers are unclear. Fortunately, there is effective treatment for asthma. Asthma drugs fall into 2 categories: drugs that control asthma over the long term, making attacks less frequent, and drugs that help during asthma attacks by relaxing the airways. Beta2-agonists are inhaled drugs that help during asthma attacks by relaxing the airways. Beta2-agonists may also have benefits related to the airway inflammation that occurs in asthma. Although beta2-agonists can improve symptoms during an asthma attack, there is concern that people who use them regularly develop tolerance to them. Tolerance means that the beta2-agonists lose some of their effectiveness.