Methandrostenolone (Dianabol) is an anabolic steroid. Methandrostenolone does not react strongly with the androgen receptor, instead relying on activity not mediated by the receptor for its effects. These include dramatic increases in protein synthesis, glycogenolysis, and muscle strength over a short space of time. However, due to its mode of action, it decreases the rate of cell respiration and decreases production of red blood cells. In high doses (30 mg or more per day), side effects such as gynaecomastia, high blood pressure, acne and male pattern baldness may begin to occur. The drug causes severe masculinising effects in women even at low doses. In addition, it is metabolised into estradiol by aromatase. This means that without the administration of aromatase inhibitors such as Anastrozole or Aminoglutethimide, estrogenic effects will appear over time in men. Many users will combat the estrogenic side effects with Nolvadex or Clomid. In addition, as with other 17alpha-alkylated steroids, the use of methandrostenolone over extended periods of time can result in liver damage without appropriate care.

The 17alpha-methylation of the steroid does allow it to pass through the liver without being broken down (hence causing the aforementioned damage to the liver) allowing it to be taken orally. It also has the effect of decreasing the steroid's affinity for sex hormone binding globulin, a protein that de-activates steroid molecules and prevents them from further reactions with the body. As a result, methandrostenolone is significantly more active than an equivalent quantity of testosterone, resulting in rapid growth of muscle tissue. However, the concomitant elevation in estrogen levels - a result of the aromatization of methandrostenolone - results in significant water retention. This gives the appearance of great gains in mass and strength, which prove to be temporary once the steroid is discontinued and water weight drops. Because of this, it is often used by bodybuilders only at the start of a "steroid cycle", to facilitate rapid strength increases and the appearance of great size, while compounds such as testosterone or nandrolone with long acting esters build up in the body to an appreciable amount capable of supporting anabolic function on their own.