β2 agonists can be divided into 4 major categories according to their rapid onset and duration of maintenance: 1. slow onset and short duration of maintenance, such as salbutamol tablets and terbutaline sulfate tablets. 2. rapid onset and short duration of maintenance, such as salbutamol aerosol and terbutaline sulfate aerosol. 3. slow onset and long duration of maintenance, such as salmeterol inhaler. 4. rapid onset and long duration of maintenance, such as formoterol inhaler. The effect is rapid and the duration of action is long, such as formoterol inhalation. The incidence of adverse reactions such as palpitations and skeletal muscle tremor is obvious when β2 agonists are used orally or intravenously in clinical practice. Therefore, the inhalation of β2 agonist-based, its adverse reactions significantly decreased.