Wof b2 agonist bronchodilators is given by inhalation, and is suitable for both terminating asthma attacks as well as for twice daily prophylaxis:

Only long acting drugs can be used as BD dose. Formoterol:- A long acting selective b2 agonist which acts for 12 hrs when inhaled(can be used twice a day). Compared to salmeterol it has a faster onset of action. It is used on a regular morning-evening schedule for round-the-clock bronchodilatation. Salmeterol:- It is long acting selective b2 agonist with a slow onset of action; used by inhalation on a twice daily schedule for maintenance therapy and for nocturnal asthma, but not for acute symptoms. Ref:- kd tripathi; pg num:-223,224