While short-acting β₂-agonists are seen as the cornerstone of treatment as relief medication for asthma, current guidelines recommend long-acting β₂-agonists as maintenance therapy in combination with inhaled corticosteroids in patients with moderate to severe asthma, poorly controlled on present treatment. Although evidence has shown that formoterol, with its fast- and long-acting profile, is effective when used both as regular and as-needed therapy in all types of asthma, there has been some concern about the potential of β₂-agonists with long-acting profiles to produce side effects with a longer duration than seen with short-acting β₂-agonists. Also, where formoterol is used as needed, a higher total daily dose would be anticipated than when taken twice daily for regular maintenance therapy and this again has led to some concern. In a number of studies, formoterol has been shown to be well tolerated, and although systemic effects expected with this class of drugs did occur, formoterol had significantly less effect on serum potassium, pulse, blood pressure, cardiac frequency and QT interval compared with terbutaline. In addition, the duration of effects was equivalent to that observed with terbutaline and salbutamol and the relative therapeutic index of formoterol compared with salbutamol was found to be 2.5. Furthermore, studies looking at long-term use of formoterol have shown there is no reduction in bronchodilatory effect, and thus, no development of tolerance. In conclusion, formoterol is well tolerated in high doses, producing side effects typical of its class, but with a duration no longer than occurs with short-acting β₂-agonists. These observations, and the lack of tolerance development, suggest that formoterol may be appropriate treatment for patients with asthma of all types and severities on an as-needed basis or as regular treatment.