Boldenone undecylenate: what it is and how it works

Boldenone undecylenate is most often referred to by the veterinary trade name Equipoise; the drug was originally created for horses, not humans. Despite this, it has circulated in amateur bodybuilding circles for decades as a "mild" steroid. We examine the nature of this molecule, how it differs from testosterone, its mechanism of action, and why the label "mild" is misleading in this context.

What is boldenone undecylenate?

Boldenone is a synthetic anabolic-androgenic steroid (AAS) derived from testosterone. Chemically, it differs from testosterone by only a single additional double bond between the first and second carbon atoms in the A-ring. For this reason, it is also referred to in scientific literature as 1-dehydrotestosterone. This seemingly minor alteration significantly affects how the molecule interacts with the body's enzymes.

Undecylenate is an ester attached to boldenone to slow its release from the injection site. Without the ester, boldenone—like testosterone—would be rapidly metabolized by the liver. With its long ester "tail," the substance forms an oil depot in the muscle, from which it is gradually released into the bloodstream over an extended period.

A key feature of the drug is its veterinary origin. Its most well-known form, Equipoise, is approved for use in horses—specifically to improve appetite, coat condition, and body mass in emaciated animals. There are no currently approved pharmaceutical formulations of boldenone for human use; therefore, any human use occurs outside of medical indications.

It is also worth noting that boldenone can be detected endogenously in the bodies of certain animals and, in trace amounts, in humans. This is significant for anti-doping and veterinary testing purposes, but it in no way renders pharmacological doses of the drug "natural" or safe.

Molecular structure and the long ester

Undecylenic acid contains eleven carbon atoms and a terminal double bond. In terms of chain length, undecylenate is similar to testosterone undecanoate; consequently, boldenone undecylenate is classified as a long-acting drug. Precise pharmacokinetic parameters for humans have not been established in controlled studies, as the drug has never undergone modern clinical trials involving human subjects.

Calculations of molecular weights show that approximately 63% of the mass of boldenone undecylenate consists of the boldenone molecule itself, with the remainder being the ester. By comparison, the pure hormone accounts for about 72% of testosterone enanthate. This is a matter of simple chemistry rather than an indicator of "potency": the ester merely determines the release rate, not the hormone's activity.

The oil base of veterinary formulations and the long ester imply another practical consideration: the drug circulates in the body for much longer than the duration of the individual injection's effect. This is significant regarding side effects—which cannot be quickly "switched off"—and regarding anti-doping control, where metabolites remain detectable for an extended period.

Veterinary drugs are manufactured according to standards for animals, whereas products from underground laboratories are often sold on the illicit market under the name "boldenone." Analyses of seized samples, regularly published by anti-doping and toxicology laboratories, show that the active ingredient content in such products often does not match the label.

How the drug works in the body

After injection, blood esterases gradually cleave the ester, and free boldenone binds to androgen receptors in muscle and other tissues. Like other AAS, it triggers the transcription of genes related to protein synthesis, nitrogen retention, and muscle fiber growth. We described this process in more detail in a separate article on the mechanism of action.

Part of boldenone is converted to estradiol by the enzyme aromatase, although, according to available reviews, more slowly than testosterone. The other part, under the action of 5α-reductase, forms dihydroboldenone, an androgen that is an active steroid in itself. Therefore, a "pure anabolic" without androgenic and estrogenic effects cannot be obtained from boldenone.

Like any exogenous androgen, boldenone suppresses the hypothalamus-pituitary-testes axis. A decrease in luteinizing and follicle-stimulating hormones leads to a drop in own testosterone and inhibition of spermatogenesis. Because of the long ester, this effect persists for a long time after the last injection.

Among bodybuilders, there is a common belief that boldenone particularly strongly stimulates the formation of erythrocytes. Stimulation of erythropoiesis is indeed a general androgenic property well demonstrated for testosterone, but no direct controlled studies quantitatively comparing boldenone and testosterone in humans have been found.

Legal and sports status

Boldenone is explicitly listed on the World Anti-Doping Agency (WADA) Prohibited List in Category S1 Anabolic Agents and is prohibited both in-competition and out-of-competition. Detection of its metabolites in an athlete's sample is a violation of anti-doping rules, unless endogenous origin is proven.

Since small amounts of boldenone can be produced naturally in the body, laboratories use isotope mass spectrometry (IRMS) for confirmation, which allows distinguishing the synthetic and endogenous origin of the steroid by the ratio of carbon isotopes.

In veterinary medicine, the status of the drug is also different. In the European Union, the use of hormonal growth stimulants in productive animals is prohibited, and boldenone is included in the list of substances whose residues are controlled in meat. Use in equestrian sports is governed by the rules of the respective federations and is generally prohibited during competition.

In Ukraine, as in many other countries, the circulation of anabolic steroids is controlled by the legislation on medicinal products. The purchase of a veterinary drug for human use is not an authorized human medical use, without any guarantees of quality and safety.

Risks that are often overlooked

The reputation of a "soft" steroid rests mainly on forum reviews, not data. There are almost no controlled safety studies in humans for boldenone, so risks must be assessed based on the overall AAS profile and animal studies. The Endocrine Society summary (Pope et al., 2014) describes this profile as broad and dose-dependent.

• Heart and blood vessels: decrease in HDL, increase in pressure, risk of myocardial hypertrophy.
• Blood: increase in hematocrit and blood viscosity.
• Hormones and fertility: suppression of own testosterone, infertility, long-term recovery.
• Skin: acne, acceleration of androgenetic alopecia.
• For women: virilization, some of the manifestations of which are irreversible.
• Product quality: fakes, non-sterility, abscesses after injections.

The long ester makes all these risks "inertial": if a side effect appears, the drug remains in the body for a long time. This fundamentally distinguishes boldenone undecylenate from short-acting forms, which, at least theoretically, are excreted faster.

Finally, it is worth remembering the psycho-emotional effects of AAS — irritability, mood swings, and after withdrawal — depressive states during hormonal deficiency. Systematic reviews (Kanayama and Pope) emphasize that some users develop dependence on anabolic steroids.

A separate layer of risks is associated with the very method of obtaining the drug. Veterinary vials are designed for animals, and the products of underground manufacturers do not pass any sterility control. Complications at the injection site — from painful infiltrates to abscesses that require surgical intervention — are described in the clinical literature specifically in users of illegal steroids.

Editorial conclusions

Boldenone Undecylenate is a veterinary anabolic steroid derived from testosterone with an additional double bond in the A ring and a long ester that provides a long-lasting effect.

It acts through androgen receptors, is partially aromatized to estradiol and converted to active dihydroboldenone. So the notion of a "soft" drug without side effects has no scientific basis.

The lack of clinical studies in humans means that no one has established safe doses for humans. The risks—from cardiovascular to long-term hormonal suppression—should be considered at least the same as for other injectable AAS.

For a deeper understanding of the topic, we recommend our materials on the mechanism of action of boldenone, on its pharmacokinetics and on the effect on the cardiovascular system.

References

1. Kicman AT. Pharmacology of anabolic steroids. Br J Pharmacol. 2008;154(3):502–521.
2. Pope HG Jr, Wood RI, Rogol A, et al. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341–375.
3. Schänzer W. Metabolism of anabolic androgenic steroids. Clin Chem. 1996;42(7):1001–1020.
4. De Brabander HF, Poelmans S, Schilt R, et al. Presence and metabolism of the anabolic steroid boldenone in various animal species: a review. Food Addit Contam. 2004;21(6):515–525.
5. World Anti-Doping Agency. The Prohibited List. Montreal: WADA; оновлюється щороку.
6. Kanayama G, Pope HG Jr. History and epidemiology of anabolic androgens in athletes and non-athletes. Mol Cell Endocrinol. 2018;464:4–13.