OBJECTIVE: Assess the effect of melatonin (5 mg) compared with placebo as an adjuvant treatment along with current behavioral and pharmacotherapy for 28 days on weekly self-reported severity of anxiety, depression, stress, and sleep complaints, and also how sleep is affecting daily life in males 18 years of age and older in recovery from substance use at a residential program in south-western Pennsylvania. BACKGROUND: Individuals in recovery experience a variety of symptoms including, but are not limited to, anxiety, depression, sleep difficulties, and stress. In the U.S., melatonin is a readily available nutraceutical that is used to alleviate sleep difficulties. Studies also suggest that melatonin may also have anxiolytic and antidepressive actions alone, as well as in those with co-morbid insomnia. Observation of clinicians treating individuals during and/or post drug cessation indicated that melatonin is commonly provided specifically to alleviate sleep difficulties with little evidence regarding efficacy in this population. The paucity of evidence as well as observation of clinical practices provided the rationale for this randomized clinical trial. METHODS: A single-center, randomized, double-blind, placebo-controlled, parallel-group trial was conducted. Seventy individuals were enrolled, block-randomized with an allocation ratio of 1:1. Intention-to-treat analysis was performed for all primary outcome measures. Primary outcome measures were assessed with the Generalized Anxiety Disorder Scale (GAD-7), Personal Health Questionnaire Depression Scale (PHQ-8), Perceived Stress Scale (PSS-14), and Pittsburgh Sleep Symptom Questionnaire-Insomnia (PSSQ-1). Secondary outcome measures were to acquire participant characteristics, determine adherence, and document adverse events. RESULTS: No statistically significant between-group differences were detected for baseline characteristics. Even though the proportion of individuals reporting an adverse event between groups was not significantly different, the frequency of reported adverse events was greater in the melatonin group. Intention-to-treat analysis for all the measured outcomes revealed no statistically significant between-group differences for same day comparisons. CONCLUSIONS: The diversity of medication regimens, and also the services provided by the residential treatment site add to the complexity of assessing the efficacy of melatonin on the measured outcomes. Given these limitations, there exists insufficient evidence to suggest that the effect of melatonin and placebo on the outcomes were significantly different.