Injectable Testosterone: From Every-Few-Days Propionate to the 10-Week Shot

Testosterone taken by mouth is largely inactivated by the liver before it reaches the circulation.[1] Every injectable testosterone developed since the molecule became available in 1935 is a response to that single fact, and the history of injections is really the history of one chemical trick applied with increasing patience.

The trick

Attach a fatty acid chain — an ester — to the testosterone molecule, and suspend it in oil. Injected into muscle, the ester slows release from the oil depot, and enzymes in the body cleave it off to liberate active testosterone.

Longer ester chain, slower release, longer interval between injections. That is the entire design principle, and everything below is an application of it.

Propionate: the short chain, and the every-few-days problem

The first injectable testosterone used a short ester and had to be given every few days to maintain concentrations. It worked, in the narrow sense that it delivered testosterone. As a therapy for a lifelong condition it was punishing, and the burden is covered in the first injectable testosterone.

Enanthate: the fifty-year default

By the 1950s, longer-acting injectable testosterone enanthate had become the preferred therapeutic modality.[1] It held that position for decades — not because its pharmacokinetics were good, but because they were tolerable and nothing better existed.

The pharmacokinetic problem with enanthate is inherent to the depot approach rather than a flaw in the drug. Concentrations peak in the days after the injection and fall until the next one, so a man on a fixed interval spends part of every cycle above his target range and part below it. That pattern, and what it feels like, is set out in the peak and crash problem and in enanthate ruled for fifty years.

Undecanoate: a longer chain, and a real improvement

Testosterone undecanoate carries a longer ester chain. Compared directly against enanthate, injectable undecanoate was found to have more favourable pharmacokinetics and pharmacodynamics.[2]

Phase I work established intramuscular undecanoate as a treatment for male hypogonadism,[3] and by 2007 there was more than eight years of accumulated clinical experience with the long-acting parenteral preparation.[4] The practical result is dosing intervals measured in weeks rather than days.

That is a genuine advance on the burden axis. It does not abolish the underlying shape of depot dosing — a longer interval means a longer curve, not a flat one.

The other direction: smaller and more often

The alternative to extending the interval is shortening it. Subcutaneous administration, in smaller and more frequent doses, was found to produce serum testosterone concentrations that remain stable between injections.[5]

This is the same insight from the opposite end. If the problem is the peak-to-trough swing, you can either stretch the ester to flatten the fall, or dose more often so the swing has less room to develop. Weekly and sub-weekly regimens are covered in daily vs weekly dosing.

What injections never solved

Two constraints survived every iteration.

1. It is an injection. Adherence to a therapy that requires needles differs from adherence to one that does not, and men discontinue testosterone therapy more often than trial data suggests — the subject of why men quit in year one.
2. A depot is a curve. Any dose delivered as a bolus and released over time has a peak and a trough. The interval decides how far apart they sit; it does not remove them.

Transdermal delivery is a different answer to the same original problem — it bypasses the liver without a depot at all, which is why it permits daily dosing. When a permeation-enhanced transdermal system was compared directly against bi-weekly enanthate injections, the comparison was specifically about pharmacokinetics, efficacy and safety.[6] That arc continues in how delivery methods evolved.

Bottom line

Injectable testosterone got better by getting slower. Propionate needed dosing every few days; enanthate stretched that to weeks and held the market for fifty years; undecanoate stretched it further.

What none of them changed is that a depot injection produces a peak and a trough, and that the interval only sets the distance between them. Every subsequent delivery route — patches, gels, sprays — exists because that shape was worth engineering around.

References

6 sources

1. Nieschlag E, Nieschlag S. Testosterone deficiency: a historical perspective. Asian J Androl. 2014;16(2):161–168. doi:10.4103/1008-682X.122358 · PMID 24435052
2. 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
3. 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
4. Saad F, Kamischke A, Yassin A, et al. More than eight years' hands-on experience with the novel long-acting parenteral testosterone undecanoate. Asian J Androl. 2007;9(3):291–297. doi:10.1111/j.1745-7262.2007.00275.x · PMID 17486268
5. 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
6. 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