Intramuscular injection is the default and has the most data. Subcutaneous injection of the same esters gives flatter levels and, in comparisons, lower hematocrit and estradiol. Gels avoid needles but absorb unevenly and transfer to partners and children. Oral undecanoate works via the lymphatics and raises blood pressure. Nasal testosterone barely suppresses LH and FSH, which preserves fertility. Pellets are set-and-forget with no way to adjust.
- In a comparison of hypogonadal men, subcutaneous testosterone enanthate produced lower post-treatment hematocrit and estradiol than intramuscular cypionate after adjustment for covariates.
- Weekly subcutaneous injections of testosterone esters kept serum testosterone stable between injections in the Journal of the Endocrine Society study.
- Nasal testosterone gel raised hematocrit less than intramuscular cypionate in a randomised trial, and its short pulses spare LH and FSH.
- Oral testosterone undecanoate carries a US boxed warning for blood pressure increases.
- Transdermal gel is the route most associated with secondary exposure of partners and children through skin contact.
The routes side by side
| Route | Level stability | Hematocrit / estradiol | Fertility while on | Practicality |
|---|---|---|---|---|
| Intramuscular ester | Depends on frequency; weekly = ~2:1 swing | Highest peaks, highest rise | Suppressed | Most data; needs technique, sites, rotation |
| Subcutaneous ester | Flattest of the injectables | Lower than IM in comparisons | Suppressed | Small needle, less soreness, more frequent |
| Transdermal gel | Daily, but absorption varies several-fold | Moderate | Suppressed | No needles; transfer risk; showering and sweat interfere |
| Oral undecanoate | Twice daily with fat; short peaks | Blood pressure rises; boxed warning | Suppressed | No needles; expensive; food-dependent |
| Nasal gel | Two or three short pulses a day | Lowest hematocrit rise in trial | Largely preserved | Nasal irritation; frequent dosing |
| Pellets | Very flat for months | Moderate | Suppressed | No adjustment once implanted; site problems |
Intramuscular
The reference route. Oil depot in muscle, slow release, full absorption. Deep injection into glute, ventrogluteal, delt or lateral quad, rotated. The swing between injections is decided by frequency, as the ester article explains. Downsides: technique matters, larger needles, soreness, scar tissue with poor rotation, and the highest peaks of any route, which is where the hematocrit and aromatisation load comes from.
Subcutaneous
The same ester and oil injected into the fat of the abdomen or thigh with an insulin-type needle. The depot releases more evenly, so levels are flatter. Clinical data: an FDA-approved weekly subcutaneous enanthate auto-injector studied over 52 weeks; a cohort in which weekly subcutaneous cypionate held stable levels between injections; and a comparison finding lower hematocrit and estradiol than intramuscular cypionate. Practical limits: small volumes per site (typically up to about 0.5–1 mL), so larger weekly amounts are split into more injections, which is exactly what flat levels want anyway. Occasional lumps or irritation at the site; rotate.
Transdermal gel
Applied daily to shoulders, upper arms or abdomen. No needles, physiological daily rhythm. Two real problems: absorption varies several-fold between people and day to day (sweat, showering, skin site), so levels are unpredictable; and the gel transfers by skin contact to partners and children until washed off, with documented virilisation of exposed children. Cover the site, wash hands, shower before contact.
Oral testosterone undecanoate
Modern capsules absorbed through the lymphatics with a fatty meal, bypassing the liver’s first pass. Twice-daily dosing, short peaks. The trials showed blood pressure increases large enough for a boxed warning in the US. Levels depend on the fat content of the meal. Not to be confused with old oral methyltestosterone, which is alkylated and liver-toxic.
Nasal
Small gel doses into each nostril two or three times a day. Short pulses, no depot, so LH and FSH are only briefly suppressed and sperm production is largely maintained, which no depot route achieves. The randomised trial against intramuscular cypionate found a smaller hematocrit rise. Costs: nasal irritation, frequent dosing, and levels that fall quickly if a dose is missed.
Pellets
Crystalline testosterone implanted under the skin of the hip under local anaesthetic, releasing over three to six months. Very flat levels, nothing to remember. But the dose cannot be changed once in, pellets can extrude or the site can get infected, and the last weeks trail off.
Choosing
- Flattest levels with a needle: subcutaneous, frequent.
- Most data and largest single doses per injection: intramuscular.
- Fertility preserved: nasal.
- No needles at all and you can manage transfer: gel.
- Blood pressure already an issue: not oral undecanoate.
Whatever the route, the three numbers to watch are the same: blood pressure, hematocrit, HDL. What to test and when.
Frequently asked questions
Is subcutaneous injection of testosterone legitimate?
Yes. A subcutaneous enanthate auto-injector is FDA-approved and studied over 52 weeks; weekly subcutaneous cypionate held stable serum levels in a published cohort. Smaller needles, flatter levels, less soreness than deep intramuscular.
Does subcutaneous absorb as well as intramuscular?
Bioavailability is comparable; the release is slower and steadier from fat than from muscle. Levels at the same weekly amount are similar on average with a smaller peak.
Why do gels vary so much?
Absorption depends on skin site, sweating, showering, and hair. Serum levels can differ several-fold between people at the same dose, and the gel transfers to anyone you touch until it is dry and washed.
Which route keeps fertility?
Nasal testosterone, because its short pulses do not suppress LH and FSH the way depot injections do. Every depot route suppresses sperm production while in use.
What about pellets?
Implanted under the skin every 3–6 months. Steady levels, no daily effort, but no dose adjustment once in, extrusion and infection risk at the site, and a slow decline at the end.
Sources
- Bhasin S et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. JCEM, 2018.
- Comparison of outcomes for hypogonadal men treated with intramuscular testosterone cypionate versus subcutaneous testosterone enanthate. 2021.
- Serum testosterone concentrations remain stable between injections in patients receiving subcutaneous testosterone. Journal of the Endocrine Society, 2017.
- Comparison of hematocrit change in testosterone-deficient men treated with intranasal testosterone gel vs intramuscular testosterone cypionate: a randomized clinical trial. 2023.
- A 52-week study of dose-adjusted subcutaneous testosterone enanthate in oil self-administered via disposable auto-injector. 2018.