Critically ill patients experience major insults that lead to increased protein catabolism. Hypermetabolism occurs early and rapidly during the first week of critical illness to provide amino acids for the production of energy via gluconeogenesis, and also for the synthesis of acute phase proteins and repair of tissue damage. During acute phase, neuroendocrine and inflammatory responses promote protein breakdown and amino acid release. Under stress conditions, protein synthesis cannot match the increased rate of muscle proteolysis because of a state of anabolism resistance, which limits uptake of amino acids into muscles. Hypermetabolism results in a significant loss of lean body mass with an impact on weaning from the ventilator and muscle recovery. Functional disability can be long term sometimes with no full return to normal. In critically ill patients, severe and persistent testosterone deficiency is very common and is observed early after Intensive Care Unit (ICU) admission. This acquired hypogonadism promotes the persistent loss of skeletal muscle protein and is related to poor outcome. Administration of testosterone induces skeletal muscle fiber hypertrophy and decreases protein breakdown in healthy young men. It has been repeatedly shown that testosterone treatment enhances muscle mass and strength in hypogonadal men and women and can improve physical performance. Testosterone administration in burned patients reduces protein breakdown and increases protein synthesis efficiency. Oxandrolone, a synthetic testosterone analogue, reduces body mass and nitrogen loss and accelerates healing in burned patients. Trials in critically ill unburned patients failed to demonstrate any effect on clinical outcome but the studies were underpowered to detect a difference. Transdermal gel testosterone is the preferred route of administration for achieving steady serum testosterone concentrations as compared to oral and intramuscular formulations. Intramuscular injection induces strong fluctuations of testosterone plasma concentrations and can cause haematoma in patients with coagulation disorders, a common condition in ICUs. Several studies have raised the concern that testosterone administration could increase the risk of cardiovascular disease events. However, in a recent meta-analysis, no significant effects on cardiovascular risk were observed with either injected or transdermal testosterone supplementation in men, and the French National Agency for Medicines (ANSM) recently reported that drugs containing testosterone were not associated with an increased risk of cardiovascular events.