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PeptidePeptides (from Ancient Greek πεπτός (peptós) ‘digested’, from πέσσειν (péssein) ‘to digest’) are short chains of amino acids linked by peptide bonds. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides , and tetrapeptides. A polypeptide is a longer, continuous, unbranched peptide chain. Hence, peptides fall under the broad chemical classes of biological polymers and oligomers, alongside nucleic acids, oligosaccharides, polysaccharides, and others. A polypeptide that contains more than approximately fifty amino acids is known as a protein. Proteins consist of one or more polypeptides arranged in a biologically functional way, often bound to ligands such as coenzymes and cofactors, or to another protein or other macromolecule such as DNA or RNA, or to complex macromolecular assemblies. Amino acids that have been incorporated into peptides are termed residues. A water molecule is released during formation of each amide bond. All peptides except cyclic peptides have an N-terminal (amine group) and C-terminal (carboxyl group) residue at the end of the peptide (as shown for the tetrapeptide in the image).

Selective Androgen Receptor Modulators or SARMs are a class of androgen receptor ligands that maintain some of the desirable effects of androgens, such as preventing osteoporsis and muscle loss while reducing risks of developing prostate cancer. In the last 1990s, the first nonsteroidal SARM, an analog of bicalutamide, was discovered. They are intended to have the same kind of effects as androgenic drugs, such as anabolic-androgenic steroids, but be more selective in their action. As of early 2020, there are no SARMs which have been approved for therapeutic use by the U.S Food and Drug Administration.

Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristics such as increased muscle and bone mass, and the growth of body hair. In addition, testosterone in both sexes is involved in health and well-being, including moods, behaviour, and in the prevention of osteoporosis. Insufficient levels of testosterone in men may lead to abnormalities including frailty and bone loss. Testosterone is a steroid from the androstane class containing a ketone and a hydroxyl group at positions three and seventeen respectively. It is biosynthesized in several steps from cholesterol and is converted in the liver to inactive metabolites. It exerts its action through binding to and activation of the androgen receptor. In humans and most other vertebrates, testosterone is secreted primarily by the testicles of males[clarification needed] and, to a lesser extent, the ovaries of females. On average, in adult males, levels of testosterone are about seven to eight times as great as in adult females. As the metabolism of testosterone in males is more pronounced, the daily production is about 20 times greater in men. Females are also more sensitive to the hormone. In addition to its role as a natural hormone, testosterone is used as a medication in the treatment of hypogonadism in men and breast cancer in women. Since testosterone levels decrease as men age, testosterone is sometimes used in older men to counteract this deficiency. It is also used illicitly to enhance physique and performance, for instance in athletes. The World Anti-Doping Agency is listing it as S1 Anabolic agent substance “prohibited at all times”.

Nandrolone, also known as 19-nortestosterone, is an androgen and anabolic steroid (AAS) which is used in the form of esters such as nandrolone decanoate (brand name Deca-Durabolin) and nandrolone phenylpropionate (brand name Durabolin). Nandrolone esters are used in the treatment of anemias, cachexia (wasting syndrome), osteoporosis, breast cancer, and for other indications. They are not used by mouth and instead are given by injection into muscle or fat. Side effects of nandrolone esters include symptoms of masculinization like acne, increased hair growth, voice changes, and decreased sexual desire due to its ability to suppress endogenous testosterone synthesis while not being a sufficient androgen itself. They are synthetic androgens and anabolic steroids and hence are agonists of the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT). Nandrolone has strong anabolic effects and weak androgenic effects, which give them a mild side effect profile and make them especially suitable for use in women and children. There are metabolites of Nandrolone that act as long-lasting prodrugs in the body, such as 5α-Dihydronandrolone.

Boldenone (developmental code name RU-18761), is a naturally occurring anabolic–androgenic steroid (AAS) and the 1(2)-dehydrogenated analogue of testosterone. Boldenone itself has never been marketed; as a pharmaceutical drug, it is used as boldenone undecylenate, the undecylenate ester. Like other AAS, boldenone is an agonist of the androgen receptor (AR). The activity of boldenone is mainly anabolic, with a low androgenic potency. Boldenone will increase nitrogen retention, protein synthesis, increases appetite and stimulates the release of erythropoietin in the kidneys. Boldenone was synthesized in an attempt to create a long-acting injectable metandienone, for androgen deficiency disorders. Boldenone acts similar to metandienone with fewer adverse androgenic effects. Although commonly compared to nandrolone, boldenone lacks progesterone receptor interaction and associated progestogenic side effects.

Trenbolone is an androgen and anabolic steroid (AAS) of the nandrolone group which itself was never marketed. Trenbolone ester prodrugs, including trenbolone acetate (brand names Finajet, Finaplix, others) and trenbolone hexahydrobenzylcarbonate (brand names Parabolan, Hexabolan), are or have been marketed for veterinary and clinical use. Trenbolone acetate is used in veterinary medicine in livestock to increase muscle growth and appetite, while trenbolone hexahydrobenzylcarbonate was formerly used clinically in humans but is now no longer marketed. In addition, although it is not approved for clinical or veterinary use, trenbolone enanthate is sometimes sold on the black market under the nickname Trenabol.

Metenolone, or methenolone, is an androgen and anabolic steroid (AAS) which is used in the form of esters such as metenolone acetate (brand name Primobolan, Nibal) and metenolone enanthate (brand name Primobolan Depot, Nibal Injection). Metenolone esters are used mainly in the treatment of anemia due to bone marrow failure. Metenolone acetate is taken by mouth, while metenolone enanthate is given by injection into muscle. Side effects of metenolone esters include symptoms of masculinization like acne, increased hair growth, voice changes, and increased sexual desire. Metenolone esters are synthetic androgens and anabolic steroids and hence are agonists of the androgen receptor (AR), the biological target of androgens like testosterone and dihydrotestosterone (DHT). They have moderate anabolic effects and weak androgenic effects, as well as no estrogenic effects or risk of liver damage. Metenolone esters are androgen esters and prodrugs of metenolone in the body. Metenolone esters were introduced for medical use in the early 1960s. In addition to their medical use, metenolone esters are used to improve physique and performance. The drugs are controlled substances in many countries and so non-medical use is generally illicit. They have mostly been discontinued for medical use and have limited availability.

Drostanolone, or dromostanolone, is an anabolic–androgenic steroid (AAS) of the dihydrotestosterone (DHT) group which was never marketed. An androgen ester prodrug of drostanolone, drostanolone propionate, was formerly used in the treatment of breast cancer in women under brand names such as Drolban, Masteril, and Masteron. This has also been used non-medically for physique- or performance-enhancing purposes.