Because steroids are hormones, patients who use them for long periods of time must be carefully monitored. The most common side effects are: weight gain; thinning of the skin; upset stomach; muscle weakness in the thighs, shoulders, and neck; “masking” or hiding a fever; mood swings; insomnia; pneumonia; and increased blood sugar levels (especially in patients with diabetes). Steroids can also interact with some seizure medications, either raising or lowering the seizure medicine levels in the blood, which can affect their effectiveness. Your doctor can explain other side effects that may occur with steroid use.
The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine .