Interval dosing produces a concentration that rises after the injection and falls before the next one. That much is measured. The dose-response study reported mean nadir testosterone concentrations of 253, 306, 542, 1,345 and 2,370 ng/dL for weekly injections of 25, 50, 125, 300 and 600 mg.[3] A nadir exists at every dose; the dose sets its height.
What is not established is the second half of the familiar story — that the shape of that curve is what men are feeling. This page separates the two, because most accounts run them together.
What the pharmacology supports
Three things, each documented.
A depot injection produces a swing — and the peak is measured, not inferred. A 24-week randomised trial compared a nightly transdermal system against 200 mg testosterone enanthate intramuscularly every two weeks in 66 hypogonadal men. The injection arm produced supraphysiological levels of testosterone, bioavailable testosterone and estradiol for several days after each injection, while the transdermal arm stayed within normal physiological ranges.[1]
The consequences were measured too. Mean morning hormone levels sat within the normal range in 77–100% of transdermal patients against 19–84% of injection patients. LH was suppressed to subnormal in 31% of injection patients versus 0% transdermal. And abnormal hematocrit elevations occurred in 43.8% of the injection group against 15.4% transdermal.[1]
The broader literature agrees on the shape: all injectable testosterone esters used clinically for substitution of male hypogonadism are characterised by unfavourable pharmacokinetics, against a goal of constant physiological serum levels.[2]
Effects track concentration, not just dose. In the dose-response study, changes in fat-free mass correlated with log testosterone concentration, and changes in leg press strength, leg power, muscle volume, hemoglobin and IGF-I were all positively correlated with concentration — while fat mass and HDL cholesterol were negatively correlated.[3]
Therefore variation in concentration is capable of producing variation in effect. That is a coherent mechanism, and it is the strongest form of the argument.
What the pharmacology does not support
The jump to symptoms. And the same study is the reason to be careful, because of what did not move: sexual function, visual-spatial cognition, mood and PSA did not change significantly at any dose — across nadir concentrations spanning 253 to 2,370 ng/dL.[3]
That is a nearly tenfold range of trough levels with no significant difference in mood or sexual function. If concentration drove those symptoms in a simple way, that is not the result you would expect.
Two caveats keep this honest in the other direction: the participants were healthy young men whose sexual function and mood were already normal, leaving little room to improve, and the study ran 20 weeks.[3] It is evidence about dose-response, not a study of how hypogonadal men feel across a weekly cycle.
The randomised comparison found the split, and it is not where you would expect
This is the most useful result on the page, and it cuts both ways.
In the head-to-head trial, the smooth transdermal profile and the swinging injection profile produced different objective results — hematocrit elevations in 43.8% versus 15.4%, subnormal LH in 31% versus 0%, gynecomastia resolving in 4 of 10 versus 1 of 9.[1]
But on symptoms, both treatments maintained sexual function and mood at prior treatment levels, assessed by questionnaire, Rigiscan and the Beck Depression Inventory.[1] The men on the profile that swung did not report worse sexual function or mood than the men on the profile that did not.
So the concentration swing has measurable consequences — they are just showing up in blood counts and gonadotropins rather than in how men said they felt.
The swing is not intrinsic to injecting
Worth stating clearly, because the popular version of this story treats "injection" as the problem.
Long-acting undecanoate. Phase I studies in hypogonadal men found serum levels not exceeding the upper limit of normal, with a castor-oil half-life of 33.9 ± 4.9 days.[7]
Weekly subcutaneous cypionate. Total testosterone stayed stable and well within the normal range across the dosing interval in a small study — 627 ± 206 ng/dL.[6]
So the profile is a property of ester × route × interval, not of the needle. Biweekly intramuscular enanthate swings; other injectable regimens demonstrably do not.
Why men's reports still matter
A consistent, reproducible pattern tied to a dosing cycle is clinically meaningful information, and it has a plausible mechanism behind it.
What it is not is proof that a different schedule fixes it — and the randomised comparison that did measure mood and sexual function found both maintained on either profile.[1] The honest position: the experience is real, the mechanism is plausible, the objective differences between profiles are documented, and the symptom difference is not. The full argument is in daily vs weekly dosing.
The wrong fix
Raising the dose to lift the trough. The dose-response data show what else comes up with it: hemoglobin rises and HDL cholesterol falls as concentration increases.[3] Erythrocytosis is already the most common side effect of testosterone therapy, with published guidance to reduce or discontinue above a hematocrit of 54%.[4]
And the guideline's target is not the top of the range — it is mid-normal.[5] Chasing a higher trough by raising the peak moves two risk markers the wrong way in pursuit of an outcome the dose-response data did not find.
Frequently asked questions
Is the "crash" real? The concentration swing is real and measured — supraphysiological for several days after each injection, with a documented nadir before the next.[1][3] The randomised comparison found objective differences between profiles (hematocrit, LH) but not differences in reported mood or sexual function.[1]
Would more frequent injections help? Pharmacokinetically they narrow the swing, and weekly subcutaneous dosing has been shown to hold levels stable across the interval in a small study.[6] Whether that improves how you feel is a separate question — see daily vs weekly dosing.
Should I raise my dose? Not as a reflex. Mid-normal is the guideline target,[5] and the risks that scale with concentration are the ones being monitored — see erythrocytosis and hematocrit.
When should I have my blood drawn? Relative to your dosing schedule, as set by your clinician — otherwise the number is uninterpretable. See the 3-month panel.
Next in this series
Whatever the mechanism, the behavioural consequence is well known to anyone who runs a clinic: a lot of men do not make it past the first year.
Continue with why men quit TRT injections in year one.
References
7 sources
- Dobs AS, Meikle AW, Arver S, et al. Pharmacokinetics, Efficacy, and Safety of a Permeation-Enhanced Testosterone Transdermal System in Comparison with Bi-Weekly Injections of Testosterone Enanthate for the Treatment of Hypogonadal Men. J Clin Endocrinol Metab. 1999;84(10):3469–3478. doi:10.1210/jcem.84.10.6078 · PMID 10522982
- Partsch CJ, Weinbauer GF, Fang R, et al. Injectable testosterone undecanoate has more favourable pharmacokinetics and pharmacodynamics than testosterone enanthate. Eur J Endocrinol. 1995;132(4):514–519. doi:10.1530/eje.0.1320514 · PMID 7711892
- Bhasin S, Woodhouse L, Casaburi R, et al. Testosterone dose-response relationships in healthy young men. Am J Physiol Endocrinol Metab. 2001;281(6):E1172–E1181. doi:10.1152/ajpendo.2001.281.6.E1172 · PMID 11701431
- Agrawal P, Singh SM, Kohn T. Management of Erythrocytosis in Men Receiving Testosterone Therapy: Clinical Consultation Guide. Eur Urol Focus. 2023;9(1):20–21. doi:10.1016/j.euf.2022.10.008 · PMID 36335038
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744. doi:10.1210/jc.2018-00229 · PMID 29562364
- McFarland J, Craig W, Clarke NJ, et al. Serum Testosterone Concentrations Remain Stable Between Injections in Patients Receiving Subcutaneous Testosterone. J Endocr Soc. 2017;1(8):1095–1103. doi:10.1210/js.2017-00148 · PMID 29264562
- Behre HM, Abshagen K, Oettel M, et al. Intramuscular injection of testosterone undecanoate for the treatment of male hypogonadism: phase I studies. Eur J Endocrinol. 1999;140(5):414–419. doi:10.1530/eje.0.1400414 · PMID 10229906
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