Testosterone is the primary male sex hormone and an anabolic steroid. First isolated in the 1930s, it was quickly recognized for its muscle-building properties. During World War II, German researchers reportedly experimented with testosterone on soldiers, and by the 1950s synthetic testosterone and its derivatives were being used by athletes. Medically, testosterone was introduced for therapeutic use (e.g. testosterone propionate in the 1930s) to treat hypogonadism (low testosterone) and other conditions. Various esterified injectable forms (like enanthate and cypionate) were developed in the mid-20th century to provide longer-acting testosterone for clinical use. Testosterone remains the standard for male hormone replacement therapy (TRT) and is considered the baseline compound against which other anabolic steroids are compared.

In the United States, testosterone is a Schedule III controlled substance, as are all anabolic-androgenic steroids (AAS) due to their potential for abuse. It is available by prescription for legitimate medical uses (such as TRT) but is illegal to use without a prescription. Internationally, testosterone is likewise regulated: for example, it is a prescription-only medicine in the UK and a controlled drug in Canada and Australia. In professional sports, testosterone and its esters are banned by the World Anti-Doping Agency. Athletes with medical needs may use it only with a therapeutic use exemption, since exogenous testosterone provides an unfair performance advantage.

Testosterone for performance enhancement is typically administered as an injectable ester. Common forms include Testosterone Enanthate, Cypionate, and Propionate, which differ in half-life and injection frequency. For example, enanthate and cypionate have half-lives around 5–7 days and are usually injected 1–2 times per week, whereas propionate acts faster (half-life ~2 days) and is injected every other day. In bodybuilding, a classic beginner cycle is 500 mg per week of a long-acting testosterone (split into two 250 mg injections) for 10–12 weeks. Experienced users may use higher doses (750–1000+ mg/week), but 500 mg is considered both effective and relatively safe as a starting point. Testosterone can also be administered in other forms (transdermal gels, pellets, oral undecanoate), but these are used medically rather than in bodybuilding. Injectable testosterone is often used as the “base” of steroid cycles – it helps maintain normal physiological functions and synergizes with other compounds.

Comparative Ester Table: Below is a comparison of common testosterone esters used in bodybuilding:

Note: All forms provide the same hormone once cleaved; differences are in release timing. Regardless of ester, exogenous testosterone aromatizes significantly into estrogen and can convert to DHT, which influences the side effect profile.

Users often report that a testosterone cycle produces a general sense of well-being, confidence, and increased energy. In the gym, testosterone greatly enhances muscle strength and training performance. Lifters find their strength increases steadily on moderate doses of testosterone, improving their capacity to lift heavier weights or perform more reps. Accompanying the strength gains is usually increased muscle mass – testosterone is highly anabolic, promoting protein synthesis and muscle fiber growth. Many also experience enhanced recovery, meaning reduced soreness and faster bounce-back between workouts. Libido often surges on-cycle (testosterone is the primary driver of sex drive), and users facetiously refer to being “on test” as feeling more virile or sexually charged. Some report mood elevation and assertiveness: “alpha” feelings both in and out of the gym. Aggression can mildly increase, which can be channeled into training intensity (though “roid rage” is generally exaggerated).

Water retention is a notable effect at higher doses due to aromatization into estrogen. This can lead to a fuller, bloated look (puffy muscles and sometimes a “moon face” puffiness). On moderate doses, the estrogen-related water weight can improve joint lubrication and strength, but excessive estrogen may cause discomfort (bloating, blood pressure rise). Overall, when used responsibly, testosterone is often described as making the user feel “on” – a heightened version of themselves with more strength, stamina, and confidence, as long as side effects are managed.

As a parent hormone, testosterone’s side effect profile is well-understood. Key side effects fall into several categories:

Estrogenic: Testosterone readily converts to estrogen (estradiol) via the aromatase enzyme. High estrogen can cause gynecomastia (breast tissue growth) and significant water retention, as well as blood pressure increases. Many users employ an aromatase inhibitor (AI) during cycles to mitigate estrogenic effects, though careful dosing of testosterone can often minimize these issues. Some estrogen is beneficial for anabolic gains and libido, so the goal is to prevent excessive estrogen, not eliminate it.

Androgenic: Testosterone is both anabolic and androgenic (rated 100/100 by convention). It can cause acne, oily skin, and accelerated male-pattern baldness in those genetically predisposed. These effects come from testosterone’s direct action and its conversion to DHT (dihydrotestosterone) via 5-alpha reductase, especially in the skin and scalp. Androgenic effects also include body hair growth and potential prostate enlargement with long-term use. Women who take testosterone (or high doses of other AAS) can suffer virilization: deepening of the voice, clitoral enlargement, and menstrual disturbances – hence testosterone is not recommended for female performance enhancement.

Cardiovascular: Testosterone can negatively impact cholesterol, typically reducing HDL (“good” cholesterol) and raising LDL (“bad” cholesterol). This effect is dose-dependent and exacerbated by aromatase inhibitors or orals. Water retention can raise blood pressure. However, among steroids, moderate testosterone is considered relatively heart-friendly (especially compared to certain orals or very androgenic drugs), and maintaining estrogen within normal range actually supports cardiovascular health. Regular blood work on-cycle often shows increased red blood cell count (testosterone stimulates erythropoiesis), which can improve endurance to a point but also increase blood viscosity.

Hepatotoxicity: Injectable testosterone is not C-17 alpha alkylated and thus not liver-toxic in the way oral steroids are. Even high doses of injectable testosterone have minimal direct liver stress. That said, extremely high doses over long periods or combining with oral AAS can elevate liver enzymes. Oral testosterone undecanoate (not commonly used in bodybuilding) bypasses some liver metabolism via lymphatic absorption, but overall testosterone is not associated with the liver damage seen in many oral steroids.

HPTA Suppression: All anabolic steroids, including testosterone, suppress the hypothalamic-pituitary-testicular axis. When exogenous testosterone is introduced, the body halts natural testosterone production. Testicular atrophy (shrinkage) occurs during cycle due to this shutdown. Without proper post-cycle therapy (PCT), it can take months for natural testosterone to recover, and long cycles or high doses increase the risk of prolonged suppression. Many users include hCG on cycle to mitigate testicular shrinkage and help maintain function.

Other: Elevated testosterone can increase aggression and irritability in some users, though this varies (often correlated with individual temperament and estrogen control). Sleep apnea or snoring can worsen due to weight/water gain. Some users report increased appetite, while others might experience acid reflux or minor digestive issues if eating large quantities to capitalize on muscle gains. Finally, anecdotally, high-dose testosterone may cause a slight increase in hematocrit which, if unchecked, can contribute to lethargy or headaches; regular blood donations are sometimes done to alleviate this.

Overall, testosterone’s side effects are dose-dependent and often manageable. Many in the bodybuilding community view moderate testosterone as the foundation of safer steroid use, noting that “most people do not experience any long-term adverse effects” when using reasonable doses responsibly – especially compared to designer steroids. Nonetheless, it requires respect: monitoring of blood pressure, cholesterol, and estrogen is important during cycles to mitigate risks.

Bulking: Testosterone (especially long esters like enanthate or cypionate) is a core bulking steroid. At 500 mg/week and above, it provides dramatic increases in muscle protein synthesis, appetite, and training recovery, enabling users to eat big and train hard. It’s often stacked with a powerful oral like Dianabol or Anadrol for synergy – the classic “test and dbol” bulk has built many physiques. The downside is bulking doses can cause notable water retention; however, this can be managed and some water is thought to aid strength and joint comfort during heavy lifting. Most first-time cycles are simple testosterone-only bulks, yielding 10–20 lbs of weight gain (a significant portion being muscle, along with some water/fat).

Cutting: Testosterone is also used in cutting phases to preserve muscle mass while dieting. A lower dose (e.g. TRT dose or moderately above, 150–250 mg/week) is sometimes employed in contest prep to maintain muscle without excessive estrogen-related bloat. Others run a full hormone replacement dose of test alongside dryer compounds (like Tren, Masteron, or Winstrol) to keep a normal physiological function and prevent muscle loss. Because testosterone can aromatize, cutting cycles that include high testosterone might require more diligent estrogen management to keep the physique dry and conditioned. Some advanced competitors actually lower testosterone and rely more on non-aromatizing steroids during the final cutting stage for maximum hardness, using just enough test to function. In any case, even during cutting, having some testosterone in the stack is valued for strength and anti-catabolic effects.

Stacking: Testosterone is famously versatile and stacks well with any anabolic steroid. It is commonly said “Test is best” as a base. For example, a Test + Deca + Dbol stack is a classic offseason mass cycle: testosterone provides the base androgen and anabolic effect, nandrolone (Deca) adds additional anabolic activity with joint comfort, and Dianabol kick-starts rapid gains. In a strength or powerlifting context, testosterone might be combined with Trenbolone or Halotestin for extreme strength gains. In cutting, Test + Tren + Masteron is a potent combination for advanced users (each addresses different pathways: test for overall mass, tren for recomposition, masteron for hardness and estrogen control). Testosterone’s presence in a stack also helps maintain libido and normal physiological function that might otherwise be suppressed by certain compounds (for instance, Deca or Tren alone can cause sexual dysfunction – having test in the mix prevents the dreaded “Deca dick” or low libido issues). It’s generally recommended that any steroid cycle include at least a TRT-level dose of testosterone to avoid problems stemming from testosterone suppression.

Cruise/Test Base: Outside of finite “cycles,” many users practice a Blast and Cruise protocol – blasting higher doses then cruising on a low dose of testosterone (e.g. 150 mg/week) instead of coming off completely. This cruise is essentially a perpetual TRT to maintain gains and avoid hormonal crashes. While not a medically sanctioned practice, it underlines how testosterone is central to PED regimens. Even those on cycle with multiple compounds often say “the test base” is non-negotiable for the reasons mentioned (muscle maintenance, libido, overall well-being).

(See the Dosing and Administration section for a detailed comparison table of testosterone esters.)