We carried out a pilot double-blind, randomized, controlled study of low-dose naltrexone (LDN) 1 mg b.i.d. versus placebo augmentation in MDD patients who relapsed on dopaminergic agents. The primary aim was to test the hypothesis that patients experiencing depressive breakthrough would demonstrate greater improvement in their depression when supplementing their current antidepressant regimen with LDN versus placebo, with no significant difference in side effects.