Testosterone Cypionate is the longest-estered testosterone available today. It has a half-life in the body of 15 to 16 days and is found as injectable oil. Because it reacts in the body for so long, it can cause more water retention than other steroids and is best used as bulking compound in a stack. Testosterone is the male sex hormone responsible for many of the physical and emotional traits of men. It creates the deeper voices, sex drive, aggression and, of coarse, larger muscles in men. Testosterone makes the muscles retain more nitrogen, which in turn makes the muscle synthesize proteins better, producing larger muscles.