Classification of Anabolic Agents

Anabolic agents are usually put into four categories: steroid hormones, synthetic hormones, xenobiotic compounds, and other growth promoting agents (GPA's). Another classification approach is: natural steroids (e.g.estradiol), artificial steroids (e.g. trenbolone), and non-steroid compounds (e.g. zeranol).

Steroid Hormones

Steroid hormones include the estrogenic (female) and androgenic (male) hormones. They are normally produced naturally in nearly all vertebrates, especially in the gonads, the adrenal glands and placenta. During pregnancy the placenta becomes the main source of estrogen. Testosterone and estrogen generate sexual characteristics, maintain reproduction, and stimulate growth. Three of the five anabolic agents approved in the U.S. are steroid hormones: estradiol, testosterone, and progesterone.3

Synthetic Hormones

Synthetic hormones such as diethylstilbestrol (DES), hexosterol, and melengestrol acetate (MGA), are analogues of endogenous steroid hormones and produce similar effects. Some of these compounds can have adverse effects on treated animals, and leave residues in meat and excreta. That is the reason why, with the exception of MGA, the others were not approved or were banned. Trenbolone acetate (TBA) is a widely used steroid with anabolic properties administered to cattle to improve weight gain, feed conversion, and nitrogen retention in many countries of the world.4

Xenobiotic Hormones

Xenobiotic hormones are derived from plants, and may not have a molecular structure related to steroidal hormones. Zeranol is the best known and most widely used of this class of hormones. It is a weak estrogen that acts by increasing the production of growth hormone, prolactin, and cortisol. Zeranol is used extensively in cattle by implanting in the ear to improve weight gain and feed efficiency.

Growth Promoting Agents

This category includes a wide variety of substances, including antibiotics. For example, Bambermycin is an antibacterial drug is used only as a growth promoting agents in veterinary medicine. It is added to cattle, swine, poultry and rabbit feeds. Lasalocid is an antibiotic widely used to control poultry coccidiosis. It has also been used as a growth promoting agent in cattle. When lasalocid is added to layer hens, considerable accumulation of drug residues occurs.5

Growth-promoting implants have been used since 1940s to increase feed efficiency and weight gain in beef cattle. Implants are small pellets or devices placed under the skin at the back of the ear (to avoid the possibility of hormone residue in food products, implants inserted in the ear are removed before slaughter). Veterinarians currently use implants of natural hormones (testosterone, estradiol, and progesterone) and synthetic hormones (TBA, MGA and zeranol) to provide a slow, sustained release of the growth promoting agent.6

Anabolic Agents in Sports

An anabolic agent is a substance that enhances the body's ability to build tissue. In regards to sports, anabolic agents are typically those that lead to an increase in muscle mass by promoting protein synthesis. The word anabolic is usually associated with anabolic steroids, which technically are steroid hormones that play a similar role to that of testosterone and are considered dietary supplements.1

The difference between anabolic steroids (AS) as doping agents and other drugs, such as amphetamines and cocaine, is that AS may be used during training sessions to produce improvements, but their use can be ceased before the competition and therefore the chance of detection in drugs tests may be reduced. Anabolic steroids are usually used in cycles. An "on-cycle" is the number of weeks for which the AS is used; an "on-cycle" is generally followed by an a period of abstinence from AS, or "off-cycle". The concomitant use of other drugs, such as masking agents, to avoid detection in drugs test, e.g. Probnecid, or drugs to ameliorate the adverse effects of AS has become part of AS-using culture.2


  1. Practical Applications in Sports Nutrition. Heather Hedrick Fink, Lisa A. Burgoon, Alan E. Mikesky
  2. Anabolic Steroids. Patrick Lenehanv
  3. Havard's Nursing Guide to Drugs. Adriana Tiziani, Margaret Havard
  4. Animal Drugs and Human Health. Lester M. Crawford, Don A. Franco
  5. Drug Residues in Foods: Pharmacology, Food Safety and Analysis. Dimitrios J. Fletouris
  6. Fundamentals of pharmacology for veterinary technicians. Janet Amundson Romich