Boldenone (Equipoise)

Boldenone, sold for animal use as Equipoise and known on the street as “EQ,” is a synthetic anabolic-androgenic steroid (AAS) — a lab-made cousin of testosterone. It is not a peptide and not a SARM. It comes as a long-acting injectable (a form called the undecylenate ester, which slows how fast the drug is released). It used to have a human version called Parenabol, but that lost its approval decades ago. Today the only approved use is in horses. The muscle and performance claims people make about it lean on how androgens (male-hormone-type drugs) work in general, plus personal stories — not on any human studies of boldenone itself.

What it is

Chemically, boldenone is just testosterone with one extra double bond added to its ring structure (the formal name is 1-dehydrotestosterone, or androsta-1,4-dien-17β-ol-3-one). The federal law lists it as “boldenone (17β-hydroxyandrost-1,4-diene-3-one).” Unlike many oral steroids, it is not 17α-alkylated — that’s a chemical tweak that lets a steroid survive being swallowed as a pill. Because boldenone lacks that tweak, it is given by injection rather than as a tablet, and it does not bring the liver damage that the swallowed 17α-alkylated steroids (like methandienone or stanozolol) are known for.

How does it work? It is a classic androgen-receptor agonist, meaning it switches on the same cell receptors that testosterone does. It mostly resists an enzyme called 5α-reductase, so relatively little of it converts into the stronger DHT-type androgen in body tissues. But it does aromatize — that is, the body can turn some of it into estrogen — so it has some estrogen-like activity. You will often see the claim that it aromatizes “at about half the rate of testosterone.” That figure comes from non-peer-reviewed bodybuilding writing, not from solid human data, so treat it as a rough guess rather than a proven number. What is fair to say is that it tends to cause less estrogen-driven water retention (bloating) than testosterone. The undecylenate form is very fat-loving and acts like a slow-release depot in the muscle, with sources reporting a long half-life (the time for half a dose to clear) of roughly 14 days after injection.

The claims

The only approved claim is for animals: the FDA-approved label is for run-down horses, to help with weight, coat, and overall condition. In non-medical human use (which is off-label and illegal), people promote boldenone for building lean muscle and strength, for boosting appetite, and for raising red blood cell counts (an effect called erythropoietic). Fans like it for producing a slow, “lean” kind of gain with less estrogen-driven bloating than testosterone. None of these human uses is backed by studies showing it actually works in people.

What the evidence actually shows

The approved use rests on an animal/veterinary muscle-building effect: in run-down horses, the approved product is labeled to improve weight, coat, and overall condition.

By today’s standards, there is essentially no human data showing it works. There are no well-run modern randomized controlled trials (the gold-standard study type, where people are randomly assigned to treatment or comparison groups) showing boldenone’s effects on muscle, strength, or any medical condition in humans. The claims about lean-mass or strength benefits in people are borrowed from how androgens work as a group, plus anecdote — not from any human trial of boldenone itself.

Most of the peer-reviewed, human-relevant research on boldenone is about catching it in drug tests, not about whether it works. There is a real body of work on spotting boldenone misuse by tracking its breakdown products (metabolites), and on telling drug-sourced boldenone apart from the tiny amounts that can show up naturally in the body or from microbes — a genuine headache for anti-doping labs. They tackle it with tools like gas chromatography–mass spectrometry, or GC–MS (a lab method that separates and identifies chemicals), and isotope-ratio mass spectrometry, or IRMS (a method that checks the carbon “fingerprint” to tell man-made from natural). The wider research on the whole AAS family does confirm that androgens really can build muscle, but it also documents the harms listed below. That is group-level evidence about steroids in general, not human proof for boldenone specifically.

Legal and regulatory status

In the United States, anabolic steroids are Schedule III controlled substances under the Controlled Substances Act. This came through the Anabolic Steroid Control Act (1990; expanded by the Anabolic Steroid Control Act of 2004, Pub. L. 108-358, signed October 22, 2004, with the steroid-definition changes taking effect in January 2005). Boldenone is named outright in the federal definition of “anabolic steroid” at 21 U.S.C. § 802(41), listed as “boldenone (17β-hydroxyandrost-1,4-diene-3-one).” So boldenone for human use is a Schedule III controlled substance, and having or sharing it without a prescription is illegal; the only legal version is the FDA-approved animal product, handled as intended. (A 2009 DEA final rule that put steroids in Schedule III is helpful background, but it adds boldione and two other steroids — not boldenone itself, which the statute schedules directly.) For comparison within this drug class: clenbuterol is not FDA-approved for humans and is not a scheduled controlled substance, and human growth hormone is covered by its own law (21 U.S.C. § 333(e)) rather than the usual scheduling — neither of those changes boldenone’s Schedule III status.

In sport, boldenone is a banned anabolic agent under Category S1 (S1.1, Anabolic Androgenic Steroids) on the WADA Prohibited List, and it is banned at all times (both in and out of competition). The 2026 List specifically covers listed AAS “and other substances with a similar chemical structure or similar biological effect(s), including their esters,” which sweeps in boldenone undecylenate. Misuse is caught by detecting boldenone and its main breakdown product, 5β-androst-1-en-17β-ol-3-one; because trace amounts can come from non-doping sources in some samples, labs use IRMS (the carbon-fingerprint test, ¹³C/¹²C) to confirm the boldenone came from outside the body.

Safety

Boldenone is a foreign androgen that acts throughout the body, and it carries the heart, hormone, and class-wide risks that come with all AAS.

• Heart and cholesterol: AAS, including non-17-alkylated injectables like this one, sharply lower HDL (“good” cholesterol) and raise LDL (“bad” cholesterol). Long-term use of high doses is linked to high blood pressure, abnormal cholesterol, hardening and early disease of the arteries, changes to the heart’s main pumping chamber and a weakened heart muscle (cardiomyopathy), irregular heartbeats, and sudden cardiac death.
• Liver (an important point that sets it apart): the classic steroid liver problems — bile blockage, blood-filled cysts (peliosis), and tumors — come from the swallowed 17α-alkylated steroids. Boldenone is not 17α-alkylated and is injected, so it is not seen as a major liver toxin the way those oral steroids are. It is still a foreign androgen that affects the whole body.
• Shutting down your own hormones and fertility: taking outside androgens suppresses the body’s own hormone-control loop (the hypothalamic-pituitary-gonadal axis, which signals the testes), lowering the messenger hormones LH and FSH. That can shrink the testicles, hurt sperm production and cause infertility, drop your natural testosterone, and after stopping leave you with low testosterone (secondary hypogonadism) along with low sex drive, erection problems, and mood changes.
• Estrogen effects: because boldenone aromatizes (converts to estrogen), breast tissue growth in men (gynecomastia) and fluid retention are possible, especially at higher doses — though usually reported as less than with testosterone.
• Masculinizing effects in women: androgenic changes such as a deeper voice, extra body hair (hirsutism), disrupted periods, and enlargement of the clitoris — some of which can be permanent.
• Blood: androgens push the body to make more red blood cells; boldenone can raise red-cell mass and hematocrit (the share of blood made up of red cells — the very effect valued in run-down horses), which in people can thicken the blood and raise the risk of clots when hematocrit gets high.
• Other class-wide effects: acne, hormone-driven hair loss, and mood or aggression changes. Illegal injecting adds risks at the injection site and from contamination or non-sterile product: the animal-grade version is not made, labeled, or quality-checked for use in humans.

Bottom line

Boldenone is an injectable anabolic-androgenic steroid, not a peptide. Its only approved use today is in animals (run-down horses), and there are no modern human clinical trials supporting its claims for muscle, strength, or performance — those rest on how androgens work in general, plus anecdote. In the US it is a Schedule III controlled substance, named outright in federal law, and it is banned at all times in sport. It does sidestep the liver toxicity tied to oral 17α-alkylated steroids, but it still carries the heart, cholesterol, hormone-suppression, estrogen, masculinizing, and blood-related risks of the whole AAS class.

Evidence grade: 3/10 · Animal only.

Sources

• 21 U.S.C. § 802(41) — federal definition of “anabolic steroid” (boldenone named)
• Anabolic Steroid Control Act of 2004, Pub. L. 108-358
• DEA final rule classifying steroids as Schedule III (general scheduling background; adds boldione and two others, not boldenone)
• FDA new-animal-drug listing for injectable boldenone (Federal Register, 2005)
• Equipoise (boldenone undecylenate) veterinary label — Drugs.com vet monograph
• Boldenone undecylenate — Wikipedia (chemistry, ester, Ciba/Squibb history, Parenabol, IM half-life)
• WADA 2026 Prohibited List (S1 Anabolic Agents; “including their esters”; prohibited at all times)
• WADA Prohibited List portal
• Gómez C et al. 2012 — new potential markers for the detection of boldenone misuse (PMID 22664392)
• Piper T et al. 2010 — IRMS ¹³C/¹²C determination of urinary boldenone and metabolite (PMID 20468009)
• de la Torre X et al. 2013 — metabolism of boldione in humans, pseudoendogenous metabolites (PMID 24259377)
• Albano GD et al. 2021 — Adverse Effects of Anabolic-Androgenic Steroids: A Literature Review, Healthcare 9(1):97 (PMC7832337)
• Cardiac and metabolic effects of AAS abuse on lipids, blood pressure, LV dimensions, and rhythm, Am J Cardiol 2010 (PMC4111565)
• Anabolic androgenic steroids may be associated with early coronary artery disease (PMC6008908)
• Anabolic Steroid Use Disorder — StatPearls (NCBI Bookshelf, NBK538174)