The pharmacokinetics of boldenone undecylenate are written confidently and with exact figures on the Internet. In fact, there are practically no modern pharmacokinetic studies of this drug in humans: most of the data were obtained in veterinary medicine and in anti-doping analysis. The editors tell us what is known for certain about the path of boldenone in the body, and what are only estimates.
Absorption: oil depot and ester
Boldenone undecylenate is administered intramuscularly in the form of an oily solution. After injection, a depot is formed in the muscle, and absorption is determined by how quickly the highly lipophilic molecule moves from the oil into the intercellular fluid, and from there into the bloodstream. This is the slowest stage, which determines the duration of the action.
Undecylenate ester has eleven carbon atoms, so it belongs to the "long" group. A general rule of thumb, well-studied for testosterone esters, is that the longer the ester, the slower the release, the lower and later the peak concentration, and the longer the substance stays in the body. It is logical to expect similar behavior from boldenone.
The rate of absorption is also affected by the injection volume, type of oil, injection site and blood supply to the muscle. These factors are demonstrated for testosterone esters in the classic pharmacokinetic works summarized in the Nieschlag and Behre monograph. There are no similar systematic data for boldenone in humans.
When the molecule enters the bloodstream, esterases cleave the ester relatively quickly. Therefore, free boldenone is mostly present in the circulation, and the ester form is only present in a small amount. The speed of hydrolysis is not a limiting factor: the "bottleneck" is the exit from the depot.
Half-life: numbers and their quality
Popular sources often quote a half-life of about two weeks for boldenone undecylenate. The editors could not find a controlled study on humans that would correctly establish this indicator. The figure is presumably an estimate by analogy with testosterone long esters rather than a direct measurement.
It is worth distinguishing between two concepts. The half-life of free boldenone from the blood is short - as with other non-ester steroids, it is hours. The "long" period discussed by users is essentially the rate of release from the depot, i.e. the so-called flip-flop kinetics, when absorption is slower than elimination.
Veterinary pharmacokinetic studies in horses exist, but their results cannot be directly transferred to humans due to differences in body weight, metabolism and dosage. Therefore, any "exact" calculations of boldenone concentrations in humans should be taken with skepticism.
Something else is more important for practice: even according to the most modest estimates, the drug remains in the body for weeks after the last injection. It is this inertia that determines the duration of side effects and suppression of the hormonal system.
Boldenone metabolism
The main organ of boldenone metabolism is the liver, although some transformations also occur in peripheral tissues. The metabolism of anabolic steroids is summarized in a review by Schänzer (1996), which is still the basic source for anti-doping analytics.
The reactions of the first phase include the oxidation of the 17β-hydroxy group with the formation of androsta-1,4-diene-3,17-dione, as well as the reduction of the C4–C5 double bond and the keto group in position 3. A characteristic feature of boldenone is the formation of 5β-reduced metabolites that retain the C1–C2 double bond.
| Stage | Process | Result |
|---|---|---|
| Hydrolysis | Cleavage of undecylenate by esterases | Free boldenone |
| Oxidation of 17β-OH | 17β-hydroxysteroid dehydrogenase | Androstadienedione |
| Reduction of ring A | 5β- and 5α-reductases | Reduced metabolites with Δ1-bond |
| Partial aromatization | Aromatase | Estradiol |
| Conjugation | Glucuronidation, sulfation | Water-soluble forms for excretion |
The main urinary metabolite used for detection is considered to be 5β-androst-1-en-17β-ol-3-one, often referred to as BM1 in the literature. Its presence, along with boldenone itself, is a standard marker of drug use.
Unlike 17α-alkylated tablet steroids, boldenone does not have a methyl group at the 17α position. Therefore, direct hepatotoxicity, characteristic of oral AAS, is less pronounced for this compound. At the same time, this does not mean complete safety for the liver, especially when combined with other substances.
Elimination and metabolite detection
After conjugation with glucuronic acid or sulfate, the metabolites become water-soluble and are excreted mainly by the kidneys with urine. A small amount can be excreted in the bile. Unchanged boldenone is present in urine only in small amounts.
Anti-doping laboratories use gas and liquid chromatography with mass spectrometry to detect boldenone and its metabolites. Due to the long ester and sensitive methods, the window of detection after application of undecylenate can be long; its exact duration depends on the dose, frequency and analytical method.
The difficulty is that low concentrations of boldenone are sometimes found in the urine of people who have not taken the drug. The described mechanisms are microbial transformation of steroids in the sample or in the intestine and consumption of animal products. Therefore, WADA requires isotope mass spectrometry (IRMS) confirmation in disputed cases.
In veterinary control of food products, the situation is similar: boldenone occurs naturally in the urine of some animals, in particular uncastrated males, which makes interpretation difficult. A review by De Brabander et al. (2004) is devoted to this.
Practical consequences for health
Slow pharmacokinetics have direct medical consequences. If a side effect appears during the drug — an increase in blood pressure, an increase in hematocrit, symptoms of estrogen excess or mood changes — the concentration of the substance in the blood will decrease for a long time after the last injection.
- Effects, including side effects, "lag" from the moment of administration.
- Side effects do not disappear quickly after discontinuation.
- Suppression of own testosterone lasts longer than with short esters.
- Metabolites remain detectable for a long time.
This same principle explains why immediate post-cessation tests cannot establish completed hormonal recovery: the results of tests during this period reflect the residual effect of the drug. Endocrinologists take this into account when planning examinations.
For women, long ester is especially dangerous: if signs of virilization appear, it is impossible to quickly stop the effect. Some changes, such as hoarseness of the voice, may become irreversible even before the drug levels decrease.
Editorial conclusions
The pharmacokinetics of boldenone undecylenate is determined by the long ester and oil depot: the drug is released slowly, and the duration of its action is limited precisely by absorption, and not by the elimination of free boldenone.
Popular half-life figures are not based on controlled studies in humans and should be considered rough guidelines rather than scientific constants.
Metabolism occurs mainly in the liver with the formation of characteristic 5β-reduced metabolites, which are excreted in the urine and are well known to anti-doping laboratories.
We also advise you to read our materials on the influence of boldenone on the own production of testosterone, on tests for health control and on the risks of virilization in women.
References
- Schänzer W. Metabolism of anabolic androgenic steroids. Clin Chem. 1996;42(7):1001–1020.
- Kicman AT. Pharmacology of anabolic steroids. Br J Pharmacol. 2008;154(3):502–521.
- 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.
- Nieschlag E, Behre HM, Nieschlag S (eds). Testosterone: Action, Deficiency, Substitution. 4th ed. Cambridge University Press; 2012.
- World Anti-Doping Agency. The Prohibited List. Montreal: WADA; оновлÑÑÑÑÑÑ ÑоÑокÑ.
- World Anti-Doping Agency. Technical Document TD IRMS: detection of synthetic forms of prohibited substances by GC/C/IRMS. Montreal: WADA.
