Many athletes use anabolic-androgenic steroids (AAS) for physical enhancement but the magnitude of these gains and associated adverse effects has not been rigorously quantified. MEDLINE, EMBASE, Cochrane, SPORTDiscus, and PsycINFO were searched to identify randomized placebo-controlled trials of AAS in healthy exercising adults that reported one of the following outcomes: muscular strength, body composition, cardiovascular endurance, or power. Two authors appraised abstracts to identify studies for full-text retrieval; these were reviewed in duplicate to identify included studies. Study quality was assessed using the Cochrane method. Data were extracted in duplicate and pooled using the DerSimonian and Laird random effects model and to calculate the ratio of mean outcome improvement where possible. Pooled standardized mean difference (SMD) in muscle strength between AAS and placebo was 0.27 (95% confidence interval, 0.07-0.47; I2 = 12.7%; 21 studies). Change in strength was 52% greater in the AAS group compared to placebo. The SMD for change in lean mass between AAS and placebo was 0.62 (95% confidence interval, 0.35-0.89; I2 = 26%; 14 studies). Due to missing data, fat mass, cardiovascular endurance, power, and adverse effects were summarized qualitatively. Only 13 of 25 studies reported adverse effects including increased low density lipoprotein (LDL), decreased high density lipoprotein (HDL), irritability, and acne. In healthy exercising adults, AAS use is associated with a small absolute increase in muscle strength and moderate increase in lean mass. However, the transparency and completeness of adverse effect reporting varied, most studies were of short duration, and doses studied may not reflect actual use by athletes.