Do some antidepressants work faster than others?

The clinical utility of antidepressant drugs is impaired by the delay in onset of their therapeutic action. It is becoming increasingly clear that differences exist between antidepressants with respect to this property, both within and between pharmacologic classes. Post hoc analyses of comparisons between selective serotonin reuptake inhibitors and dual-action antidepressants such as mirtazapine and venlafaxine indicate that the dual-action drugs may have a faster onset of action. At least in the case of mirtazapine, the earlier onset appears to be via a specific antidepressant effect and not an effect on sleep or other accessory symptoms. Studies that compare mirtazapine and venlafaxine are relatively rare and lack sufficient statistical power to determine a difference in the onset of action. Although these differences have been shown in clinical efficacy studies not specifically designed to detect differences in onset of action, a definitive demonstration of early onset of action awaits the results of appropriately designed and powered clinical studies currently planned or in progress.