Cochrane

Regular fixed-dose treatment with formoterol and inhaled corticosteroids versus regular treatment with salmeterol and inhaled corticosteroids for chronic asthma: serious adverse events
Previous reviews have found a small increase in serious adverse events with both regular formoterol and regular salmeterol in comparison to placebo treatment. This review sought information from trials that compared the two treatments head-to-head, when an inhaled corticosteroid was also given together with the formoterol and salmeterol. Eight trials on 6,163 adults and adolescents were found, but no trials on children. There was no significant difference between the treatments, but serious adverse events were too rare to be confident that the risks are the same for both treatments. Comparative safety studies in children are lacking.

En Cochrane