A weekly dose of 100 mg of testosterone enanthate or cypionate is one of the most commonly prescribed replacement regimens, and the short answer is that it does quite a lot. In hypogonadal men, this dose reliably brings serum testosterone into the normal reference range, and the downstream effects touch everything from muscle mass and strength to libido, red blood cell production, and bone density. Whether you are weighing a prescription your doctor just wrote or wondering if the dose sounds too low to matter, the clinical evidence is clear that 100 mg per week is a physiologically significant amount of hormone with measurable, well-documented effects across multiple body systems.
Where 100 mg Per Week Puts Your Blood Levels
The first thing most people want to know is whether this dose will actually raise testosterone into normal territory. It does. In a study comparing subcutaneous and intramuscular injection routes, participants receiving 100 mg per week reached steady-state total testosterone concentrations within the standard reference range of 300 to 1,100 ng/dL, without the supraphysiologic spikes seen with larger biweekly doses.1PubMed Central. Testosterone Therapy With Subcutaneous Injections: A Safe, Practical, and Reasonable Option A separate study of men on subcutaneous testosterone found an overall mean total testosterone of about 627 ng/dL between injections, with relatively stable levels throughout the week.2Journal of the Endocrine Society. Serum Testosterone Concentrations Remain Stable Between Injections in Patients Receiving Subcutaneous Testosterone That mid-range number is roughly where a healthy younger man sits naturally.
For context, the body’s own production in healthy young men runs somewhere around 5 to 10 mg of testosterone per day.3The Journal of Clinical Endocrinology & Metabolism. Mean Plasma Concentration, Metabolic Clearance and Basal Plasma Production Rates of Testosterone in Normal Young Men and Women Using a Constant Infusion Procedure4The Journal of Clinical Endocrinology & Metabolism. Plasma Production Rates of Testosterone in Normal Adult Men and Women and in Patients with the Syndrome of Feminizing Testes Injecting 100 mg once a week translates to roughly 14 mg per day on paper, but testosterone esters are not 100 percent bioavailable, and some is lost to metabolism at the injection site. What actually reaches circulation puts most men in that solidly normal zone, not sky-high and not barely scraping by.
Muscle Mass and Strength
One of the most studied effects of 100 mg per week is its impact on body composition. In a trial of seven hypogonadal men given 100 mg of testosterone enanthate weekly for ten weeks, fat-free mass increased by about 5 kg (roughly 11 lbs), and body weight rose by about 4.5 kg, with no significant change in body fat percentage.5The Journal of Clinical Endocrinology & Metabolism. Testosterone Replacement Increases Fat-Free Mass and Muscle Size in Hypogonadal Men That is a substantial shift in a short time, particularly for men whose testosterone had been markedly low. Muscle size measured by MRI also increased. In broader reviews of testosterone replacement in older men, the gains tend to be more modest, around 2 kg of lean mass on average, which still represents a meaningful improvement in functional capacity and metabolic health.6PubMed Central. The benefits and risks of testosterone replacement therapy: a review
Strength follows the same trajectory. In HIV-positive men with low testosterone, testosterone treatment alone (without a structured exercise program) increased maximum voluntary strength by roughly 17 to 28 percent across multiple exercises including leg press, bench press, and lat pulls.7JAMA. Testosterone Replacement and Resistance Exercise in HIV-Infected Men With Weight Loss and Low Testosterone Levels Those who combined testosterone with resistance training saw even larger gains. So if you are starting from a low baseline, 100 mg per week provides a strong enough hormonal signal to drive real, measurable improvements in strength even without major changes to your training.
Sexual Function and Libido
For many men considering testosterone therapy, low sex drive or erectile difficulties were the symptoms that sent them to a doctor in the first place. The evidence here is encouraging, with some nuance. Testosterone replacement consistently improves libido in men who start with genuinely low levels. In the Sexual Function sub-trial of the large Testosterone Trials, which enrolled men 65 and older with levels below 275 ng/dL, libido improved in proportion to the rise in testosterone.8PubMed Central. Testosterone Therapy Improves Erectile Function and Libido in Hypogonadal Men A meta-analysis pooling over a thousand men across fourteen trials confirmed the same pattern. Once testosterone levels normalize, though, additional increases do not keep pushing desire higher, which is one reason why going above replacement doses does not keep amplifying sexual benefits.
Erectile function also tends to improve, though the effect is less dramatic than the libido boost, especially in men whose erectile dysfunction has other contributing causes like vascular disease or diabetes. In a trial of men with type 2 diabetes, testosterone replacement improved erectile function scores, intercourse satisfaction, desire, and orgasm within as early as six weeks.9The Journal of Sexual Medicine. Testosterone Replacement Therapy with Long‒Acting Testosterone Undecanoate Improves Sexual Function and Quality‒of‒Life Parameters vs. Placebo in a Population of Men with Type 2 Diabetes One important wrinkle from that same trial: men with depression at baseline responded much less robustly to treatment on both sexual and psychological measures. If mood is a major part of the picture, testosterone alone may not be the whole answer.
Bone Density
This is an effect that rarely makes the headlines but matters a great deal over time. Testosterone is a key regulator of bone turnover in men, and prolonged hypogonadism leads to measurable bone loss. In a long-term study following 72 hypogonadal men on testosterone replacement for up to 16 years, the most dramatic increase in bone mineral density occurred during the first year of treatment, with lumbar spine density jumping from an average of about 95 to 120 mg/cm³ hydroxyapatite.10The Journal of Clinical Endocrinology & Metabolism. Long-Term Effect of Testosterone Therapy on Bone Mineral Density in Hypogonadal Men Continued therapy maintained density in the normal range regardless of patient age or whether the hypogonadism was primary or secondary. More recent reviews support the same conclusion: testosterone replacement benefits bone density, especially in men who already have thinning bones.11PubMed Central. Testosterone and Bone Health in Men: A Narrative Review
Some of this bone effect actually occurs through estradiol, because testosterone is partly converted to estrogen in the body, and estrogen plays a surprisingly large role in male bone health. That conversion is one reason clinicians monitor estradiol levels alongside testosterone during therapy.12PubMed Central. Relation of Testosterone, Dihydrotestosterone, and Estradiol With Changes in Outcomes Measures in the Testosterone Trials
What Happens to Your Own Testosterone Production
This is the trade-off that does not get enough attention in casual discussions about testosterone. When you inject testosterone from the outside, your brain reads the elevated blood levels and dials down its own signals to the testes. Specifically, the pituitary gland slashes its output of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the two hormones that tell the testes to produce testosterone and sperm.8PubMed Central. Testosterone Therapy Improves Erectile Function and Libido in Hypogonadal Men This happens at every dose, including 100 mg per week. Injectable testosterone suppresses LH by roughly 70 percent and FSH by roughly 85 percent.13PubMed. The Effect of Longer-Acting vs Shorter-Acting Testosterone Therapy on Follicle Stimulating Hormone and Luteinizing Hormone
The practical consequence is that sperm production drops substantially, and many men on TRT become functionally infertile while they are on it.14PubMed Central. Testosterone Is a Contraceptive and Should Not Be Used in Men Who Desire Fertility15PubMed Central. Understanding and managing the suppression of spermatogenesis caused by testosterone replacement therapy (TRT) and anabolic-androgenic steroids (AAS) If you are trying to conceive or think you might want to in the near future, this is a critical conversation to have with your doctor before starting. Recovery is possible after stopping: in one study of men who had been on long-acting testosterone injections, LH and FSH gradually recovered toward their pre-treatment baselines over about 12 months.16European Journal of Endocrinology. Recovery of male reproductive endocrine function after ceasing prolonged testosterone undecanoate injections But “gradually over 12 months” is not the same as “quickly and easily,” and some men experience prolonged suppression.
Red Blood Cells, Lipids, and PSA
Testosterone stimulates red blood cell production, which is both a therapeutic benefit and a monitoring concern. In one study, hemoglobin rose by about 7 to 10 percent in men receiving testosterone, driven by increased erythropoietin levels and decreased hepcidin.17PubMed Central. Testosterone Induces Erythrocytosis via Increased Erythropoietin and Suppressed Hepcidin: Evidence for a New Erythropoietin/Hemoglobin Set Point For men who were anemic due to low testosterone, this is welcome. For men whose red blood cell counts were already on the high end, the continued rise can push hematocrit above the safe range, increasing blood viscosity and, at least in theory, the risk of clotting events. This is why most prescribing guidelines call for periodic blood work including a complete blood count. The effect is dose-dependent and more pronounced in older men.18The Journal of Clinical Endocrinology & Metabolism. Effects of Graded Doses of Testosterone on Erythropoiesis in Healthy Young and Older Men
Cholesterol shifts on TRT tend to be mixed. Exogenous testosterone generally lowers HDL cholesterol, which sounds alarming, but it also tends to lower total cholesterol and LDL cholesterol at the same time.19PubMed Central. An update on testosterone, HDL and cardiovascular risk in men Whether that HDL drop translates into any meaningful cardiovascular risk remains debated. It is worth tracking lipid panels periodically, but the effect is not a clear-cut negative.
Prostate-specific antigen (PSA) tends to rise modestly when testosterone therapy begins, then levels off. In a large randomized trial, testosterone treatment increased PSA by about 0.11 to 0.15 ng/mL more than placebo over the first year. After 12 months, the difference between the treatment and placebo groups narrowed, and by later time points there was no statistically significant gap.20JAMA Network Open. Prostate Safety Events During Testosterone Replacement Therapy in Men With Hypogonadism: A Randomized Clinical Trial A meta-analysis of 26 trials found the overall PSA increase after starting TRT was only about 0.10 ng/mL.21PubMed. Endogenous and exogenous testosterone and the risk of prostate cancer and increased prostate-specific antigen (PSA) level: a meta-analysis That is a trivially small shift in absolute terms, though your doctor should still establish a baseline PSA before you start and recheck it periodically.
Hair Loss and Skin Changes
Testosterone converts partly to dihydrotestosterone (DHT), which is the androgen most responsible for male-pattern hair loss in genetically susceptible people. In a one-year cohort study of men using androgens, only about 2 percent reported hair loss at the start, but that rose to about 12 percent by the end of the cycle, while 5 percent actually reported increased scalp hair growth.22PubMed Central. Hair loss in athletic testosterone use in males: a narrative review That study involved anabolic steroid users taking doses well above replacement level, so 100 mg per week likely carries a lower risk. Still, if you carry the genetic variants that predispose to hair loss, even replacement-level testosterone can accelerate the process. Acne is another common androgenic side effect, usually mild at replacement doses but worth mentioning to your prescriber if it becomes bothersome.
Why Individual Responses Vary So Much
Two men on the same 100 mg per week protocol can have noticeably different experiences, and genetics are a big part of the reason. The androgen receptor gene contains a repeating DNA segment (a CAG repeat) whose length varies between individuals, and shorter repeats make the receptor more sensitive to testosterone’s effects. A pharmacogenetic study of hypogonadal men on testosterone replacement found that prostate growth under treatment was strongly dependent on this repeat length. Men with shorter CAG repeats had substantially greater prostate growth, with an odds ratio of nearly 9 compared to men with longer repeats for reaching a prostate volume above 30 mL.23The Journal of Clinical Endocrinology & Metabolism. Prostate Volume and Growth in Testosterone-Substituted Hypogonadal Men Are Dependent on the CAG Repeat Polymorphism of the Androgen Receptor Gene The same receptor variability likely affects muscle gains, hair loss, and other androgen-dependent outcomes, even though those haven’t been studied as extensively in a pharmacogenetic framework.
Age also makes a difference. Older men tend to experience larger increases in hematocrit on the same dose, partly because their baseline erythropoietin response is more sensitive to androgens.18The Journal of Clinical Endocrinology & Metabolism. Effects of Graded Doses of Testosterone on Erythropoiesis in Healthy Young and Older Men Body composition, existing health conditions, the degree of testosterone deficiency at baseline, and even how much body fat you carry (which affects how much testosterone converts to estradiol) all interact to shape what 100 mg per week actually does for you.
Does Injection Frequency and Route Matter at the Same Dose?
Yes, and more than you might think. Injecting 100 mg all at once produces a peak in the first day or two, followed by a gradual decline before the next shot. Some men feel that roller-coaster acutely as shifts in energy, mood, or libido. Splitting the same weekly total into two or more smaller injections flattens the curve. Clinical guidance now recognizes that more frequent injections at smaller individual doses can reduce peak-related symptoms like fluid retention, elevated blood pressure, or estradiol spikes, particularly in men with low sex hormone-binding globulin.24PubMed Central. Individualizing Injectable Testosterone Replacement Therapy in Primary Care: Pharmacokinetics, Symptom Stability, Safety Monitoring, and Injection Frequency
The subcutaneous route has also gained traction as an alternative to the traditional intramuscular injection. Studies comparing the two routes at the same dose have found comparable total testosterone exposure, with subcutaneous injections producing slightly smoother levels and causing less injection-site pain and anxiety.25American Journal of Health-System Pharmacy. Pharmacokinetics, safety, and patient acceptability of subcutaneous versus intramuscular testosterone injection for gender-affirming therapy: A pilot study The practical upside is that subcutaneous injections use smaller needles and are easier to self-administer, which lowers the barrier for men who need to inject frequently.
When 100 mg Per Week Might Not Be Enough
While 100 mg weekly is a standard starting dose, it is not a universal sweet spot. Men with higher body weight, higher levels of sex hormone-binding globulin, or more rapid metabolism of the testosterone ester may find their trough levels dropping below the therapeutic range by day six or seven. The sign is usually a return of the original symptoms (fatigue, low mood, reduced libido) in the day or two before the next injection. In those cases, the fix is usually a dose adjustment or frequency change rather than a wholesale protocol overhaul. On the other hand, men who are leaner or who metabolize the ester more slowly may find that 100 mg produces levels on the higher end, sometimes high enough to amplify side effects like elevated hematocrit or acne. This is why most prescribers check blood levels about six to eight weeks in, timed at the trough (just before the next injection), and adjust from there.
The gap between “prescribed dose” and “what actually happens in your bloodstream” is real and varies enough between individuals that treating 100 mg as a fixed answer rather than a starting point is one of the more common mistakes in TRT management. The dose is a reliable beginning, but the blood work tells the real story.