Abstract
A comprehensive scientific review of testosterone suspension, the unesterified aqueous testosterone preparation, covering its historical significance as the original injectable testosterone, unique pharmacokinetics, immediate peak kinetics, clinical applications, and comparative pharmacology.
Testosterone suspension is an aqueous preparation of unesterified (free-base) testosterone in a microcrystalline water-based vehicle. Unlike all other injectable testosterone formulations, it contains no ester modification of the testosterone molecule. The active compound is pure testosterone with the molecular formula C19H28O2 and a molecular weight of 288.42 grams per mol. Because there is no ester weight, testosterone suspension delivers 100 percent active testosterone per milligram, the highest hormone-to-weight ratio of any testosterone preparation. The CAS registry number for testosterone is 58-22-0. The IUPAC name is (8R,9S,10R,13S,14S,17S)-17-hydroxy-10,13-dimethyl-1,2,6,7,8,9,11,12,14,15,16,17-dodecahydrocyclopenta[a]phenanthren-3-one. Testosterone suspension holds the distinction of being the original form of injectable testosterone, predating all ester-modified preparations and serving as the foundational reference compound against which all subsequent testosterone formulations have been evaluated.
The historical significance of testosterone suspension is profound. Following the independent isolation and structural elucidation of testosterone by Butenandt (who crystallized it from police-collected human urine) and Ruzicka (who achieved the first partial synthesis from cholesterol) in 1935, the challenge of therapeutic delivery immediately arose. Free testosterone has a plasma half-life of only 10 to 100 minutes following intravenous injection due to rapid hepatic metabolism, predominantly via reduction and conjugation pathways. Direct oral administration is essentially ineffective because near-complete first-pass hepatic metabolism destroys the hormone before it reaches systemic circulation. The first practical solution was to suspend microcrystalline testosterone in water for intramuscular injection. The microcrystalline suspension created a depot at the injection site from which testosterone dissolved gradually into surrounding tissue fluid. This approach was used in the late 1930s and throughout the 1940s in the earliest clinical investigations of testosterone therapy, including seminal studies by Heller and Myers on male hypogonadism, by Kenyon and colleagues on the anabolic effects of testosterone, and by the Council on Pharmacy and Chemistry of the American Medical Association, which in 1939 endorsed testosterone as therapeutically effective for male hypogonadism.
The pharmacokinetic profile of testosterone suspension is uniquely characterized by rapid absorption, early peak, and swift elimination. Following intramuscular injection, the microcrystalline testosterone particles dissolve relatively quickly from the aqueous depot into surrounding tissue fluid, with systemic absorption occurring over a period of hours rather than the days to weeks typical of oil-based esterified preparations. Serum testosterone concentrations rise rapidly, reaching peak levels within 2 to 6 hours post-injection. The magnitude of the peak is substantially higher per unit dose than any esterified formulation because there is no ester hydrolysis rate-limiting step and no oil depot retarding absorption. A single 50 mg intramuscular injection of testosterone suspension can produce peak serum testosterone concentrations exceeding 2,000 to 3,000 ng/dL within hours, far exceeding the physiological range. Testosterone levels then decline rapidly, with an effective duration of action of approximately 24 to 48 hours before returning to baseline. The apparent terminal half-life of the preparation, which reflects the dissolution rate from the injection depot rather than the systemic elimination of testosterone itself, is approximately 1 day (range 12 to 36 hours depending on particle size, injection site, and individual factors).
This rapid pharmacokinetic profile means that testosterone suspension requires daily or near-daily injection to maintain therapeutic testosterone levels, the most frequent injection schedule of any testosterone formulation. Typical clinical dosing ranges from 25 to 50 mg daily for testosterone replacement, though this indication is rarely used in contemporary practice due to the availability of more convenient long-acting esters. The rapid peak and clearance produce the widest peak-to-trough fluctuation of any testosterone formulation when considered on a per-injection-cycle basis, with serum levels potentially spanning from supraphysiological peaks to hypogonadal troughs within a single 24-hour period. However, this rapid cycling also means that no single injection produces a prolonged supraphysiological exposure, which may have implications for certain adverse effects.
The formulation of testosterone suspension involves the preparation of micronized testosterone crystals (typically 5 to 50 micrometers in diameter) suspended in a sterile aqueous vehicle containing sodium chloride for isotonicity, carboxymethylcellulose sodium or povidone as suspending agents to maintain uniform particle distribution, and benzyl alcohol as a bacteriostatic preservative. Unlike oil-based preparations, the water-based vehicle of testosterone suspension produces a formulation with low viscosity that can be injected through fine-gauge needles (25 to 30 gauge) with minimal injection force. However, the microcrystalline nature of the suspension requires thorough agitation (rolling or inverting the vial, not vigorous shaking) before each use to ensure uniform particle distribution and consistent dosing. Settling of particles during storage can lead to inconsistent dosing if the vial is not properly resuspended. Some preparations use a smaller particle size to improve suspension stability and absorption uniformity, while others employ larger particles for modestly prolonged release.
Clinical indications for testosterone suspension in contemporary practice are limited compared to long-acting esterified formulations. The most relevant current clinical application is in diagnostic testing, where a short-acting testosterone preparation is needed to assess physiological response to testosterone without producing prolonged exposure. For example, a testosterone stimulation test may use a short course of testosterone suspension to evaluate whether symptoms of hypogonadism respond to testosterone, with the advantage that blood levels normalize within 48 to 72 hours if the trial is discontinued. Historically, testosterone suspension was used extensively in clinical research protocols where investigators needed precise control over the timing and magnitude of testosterone exposure, including studies of testosterone's acute effects on neurocognitive function, sexual arousal, and exercise performance. The immediate peak kinetics allow researchers to study the acute physiological effects of testosterone elevation in a way that is not possible with longer-acting preparations.
Testosterone suspension has also occupied a notable position in the history of athletic performance enhancement, predating the development of ester-modified formulations and continuing as a niche preparation used for its rapid onset of action. The immediate and pronounced elevation of serum testosterone following injection was exploited by athletes seeking pre-competition testosterone elevation with rapid clearance that might avoid detection in drug testing. However, modern anti-doping testing methodology, including the testosterone-to-epitestosterone (T/E) ratio test and isotope ratio mass spectrometry (IRMS), can detect exogenous testosterone administration regardless of the formulation used. The World Anti-Doping Agency (WADA) has included testosterone and all its esters on the Prohibited List since the inception of the WADA Code, and current carbon isotope ratio testing can distinguish synthetic testosterone from endogenous production based on subtle differences in the ratio of carbon-13 to carbon-12 isotopes arising from the synthetic manufacturing process versus biological synthesis.
Comparison with esterified testosterone preparations highlights both the unique properties and limitations of testosterone suspension. The absence of an ester means that testosterone suspension provides the fastest onset of therapeutic effect (hours versus days for short-acting esters and weeks for long-acting esters), the most rapid clearance from the system, and the highest active testosterone content per milligram of formulation. However, these pharmacokinetic characteristics also mean that testosterone suspension provides the least stable blood levels, the shortest duration of action, and the highest injection frequency requirement. The water-based vehicle differs fundamentally from the oil vehicles used for esterified preparations: aqueous suspensions are less painful at the injection site for many patients (though some report more pain due to crystal-induced local irritation), do not form the same type of slow-release depot as oil-based formulations, and have different mixing and settling characteristics that require attention to ensure accurate dosing.
Compared to testosterone propionate, the shortest-chain commonly used ester, testosterone suspension provides even faster onset (peak at 2 to 6 hours versus 12 to 36 hours for propionate) and shorter duration (24 to 48 hours versus 48 to 72 hours for propionate). The difference in peak timing means that testosterone suspension produces a more immediate and pronounced acute elevation, while propionate provides a somewhat more sustained release. For clinical applications requiring rapid testosterone elevation, suspension is superior. For applications requiring somewhat more stable levels with less frequent injection (every other day rather than daily), propionate is preferred.
Compared to testosterone cypionate and enanthate, the differences are more pronounced. These long-acting esters provide peak levels at 48 to 72 hours and sustain therapeutic levels for one to two weeks, allowing weekly or biweekly injection. The trade-off is slower onset, more prolonged exposure to supraphysiological peaks, and slower washout upon discontinuation. For routine TRT, cypionate and enanthate are overwhelmingly preferred due to the practical advantages of less frequent injection. Compared to testosterone undecanoate (Nebido/Aveed), the difference is extreme: undecanoate requires injection only every 10 to 14 weeks but takes several injection cycles to reach steady state, whereas suspension provides immediate but transient testosterone elevation with each injection.
The side effect profile of testosterone suspension shares common features with all exogenous testosterone but has some formulation-specific considerations. The rapid and pronounced peak in serum testosterone following each injection produces a correspondingly rapid and pronounced conversion to estradiol via aromatase, which can manifest as transient estrogen-related effects including fluid retention, breast tenderness, and mood changes in susceptible individuals. However, the brief duration of exposure means that chronic estrogen elevation is less likely than with longer-acting formulations that maintain sustained supraphysiological testosterone levels. Erythrocytosis risk is theoretically lower with testosterone suspension due to the absence of prolonged supraphysiological testosterone exposure, though this has not been confirmed in comparative clinical studies. Suppression of the hypothalamic-pituitary-gonadal axis occurs with testosterone suspension as with all exogenous testosterone, but recovery following discontinuation is faster than with any esterified formulation, typically occurring within one to two weeks.
Injection site-specific adverse effects with testosterone suspension include pain, swelling, and occasionally sterile abscess formation. The microcrystalline testosterone particles can cause localized crystal-induced inflammation, analogous to the mechanism of gout or pseudogout, producing injection site pain that may persist for 24 to 48 hours. This effect varies with particle size, injection volume, and individual tissue reactivity. Some aqueous testosterone preparations marketed for veterinary use contain larger particle sizes designed for slower absorption, but these formulations tend to produce more injection site irritation and are not intended for human use. Allergic reactions to excipients in the aqueous vehicle (such as carboxymethylcellulose sodium or benzyl alcohol) are rare but possible.
The evidence base for testosterone suspension, while historically extensive, is less robust in the contemporary literature compared to long-acting esterified formulations, reflecting its limited role in current clinical practice. The foundational clinical studies of testosterone therapy in the 1930s through 1950s used testosterone suspension as the primary formulation and established the fundamental evidence for testosterone's effects on sexual function, mood, body composition, bone density, and erythropoiesis. These historical studies remain important references in androgen pharmacology. More recent research has employed testosterone suspension primarily in short-term mechanistic studies investigating the acute effects of testosterone on specific physiological endpoints, rather than in long-term clinical efficacy or safety trials. For example, testosterone suspension has been used in pharmacokinetic modeling studies to characterize the baseline parameters of testosterone absorption, distribution, and clearance without the confounding variable of ester hydrolysis kinetics.
Regulatory status of testosterone suspension is consistent with other testosterone formulations. It is FDA-approved for the treatment of male hypogonadism and is classified as a Schedule III controlled substance in the United States. Commercially available preparations are limited compared to esterified formulations, and availability varies by country. Some compounding pharmacies prepare testosterone suspension formulations on a patient-specific basis. Brand names have included Aquaviron, Histerone, and Testosus, among others, though many of these have been discontinued. Storage should be at controlled room temperature (20 to 25 degrees Celsius), protected from light and freezing. Freezing can damage the microcrystalline particle structure and alter dissolution characteristics. The suspension should be gently mixed before each use to resuspend settled particles, and visual inspection should confirm a uniformly milky or opalescent appearance without clumps or aggregates. Clear areas indicating particle settling require thorough remixing before dose withdrawal. The shelf life of commercially manufactured testosterone suspension is typically 24 to 36 months when stored properly. Compounded preparations may have shorter beyond-use dates as specified by the compounding pharmacy.