Mastebolin (100 mg/ml; 10 ml vial) is a sterile injectable anabolic/androgenic steroid solution used to promote muscle and strength gains as part of a bodybuilding and fitness program. Mastebolin is a part of the class of anabolic steroids known as 19-nor compounds, which are an altered form of testosterone. The main active ingredient in Mastebolin is Metribolone (R1881), a synthetic anabolic steroid. It is moderately androgenic and has been demonstrated to increase muscular mass and strength in men. Due to its short half-life and short lived effects, frequent administration of Mastebolin is highly recommended to maintain a steady state of muscle gains. Possible side effects of Mastebolin include elevated blood pressure and an increase in estrogen levels. Women should not use Mastebolin as it may cause virilization.