Others that have also been available and used commonly but to a lesser extent include methyltestosterone, oxandrolone, mesterolone, and oxymetholone, as well as drostanolone propionate (dromostanolone propionate), metenolone (methylandrostenolone) esters (specifically metenolone acetate and metenolone enanthate), and fluoxymesterone. Dihydrotestosterone (DHT), known as androstanolone or stanolone when used medically, and its esters are also notable, although they are not widely used in medicine. Boldenone undecylenate and trenbolone acetate are used in veterinary medicine.Designer steroids are AAS that have not been approved and marketed for medical use but have been distributed through the black market. Examples of notable designer steroids include 1-testosterone (dihydroboldenone), methasterone, trenbolone enanthate, desoxymethyltestosterone, tetrahydrogestrinone, and methylstenbolone. Routes of administration
There are four common forms in which AAS are administered: oral pills; injectable steroids; creams/gels for topical application; and skin patches. Oral administration is the most convenient. Testosterone administered by mouth is rapidly absorbed, but it is largely converted to inactive metabolites, and only about one-sixth is available in active form. In order to be sufficiently active when given by mouth, testosterone derivatives are alkylated at the 17α position, e.g. methyltestosterone and fluoxymesterone. This modification reduces the livers ability to break down these compounds before they reach the systemic circulation. Testosterone can be administered parenterally, but it has more irregular prolonged absorption time and greater activity in muscle in enanthate, undecanoate, or cypionate ester form.
Aromatase is highly expressed in adipose tissue and the brain, and is also expressed significantly in skeletal muscle. 3α-HSD is highly expressed in skeletal muscle as well.Natural AAS like testosterone and DHT and synthetic AAS are analogues and are very similar structurally. For this reason, they have the capacity to bind to and be metabolized by the same steroid-metabolizing enzymes. According to the intracellular metabolism explanation, the androgenic-to-anabolic ratio of a given AR agonist is related to its capacity to be transformed by the aforementioned enzymes in conjunction with the AR activity of any resulting products. For instance, whereas the AR activity of testosterone is greatly potentiated by local conversion via 5α-reductase into DHT in tissues where 5α-reductase is expressed, an AAS that is not metabolized by 5α-reductase or is already 5α-reduced, such as DHT itself or a derivative (like mesterolone or drostanolone), would not undergo such potentiation in said tissues. Moreover, nandrolone is metabolized by 5α-reductase, but unlike the case of testosterone and DHT, the 5α-reduced metabolite of nandrolone has much lower affinity for the AR than does nandrolone itself, and this results in reduced AR activation in 5α-reductase-expressing tissues.