The First Injectable Testosterone Had to Be Given Every Few Days

The ester was invented to solve a problem the Nobel Prize did not solve. Testosterone given orally is inactivated in the liver, so parenteral routes or modifications of the molecule had to be found.[1] Attaching a fatty-acid chain and suspending the result in oil was the first workable answer: injected into muscle, the ester releases slowly and the liver is bypassed entirely.

Propionate has a short chain. Short chain, fast release, brief duration — which is why the maintenance schedule it demanded was measured in days rather than weeks.

How an ester buys time

Plain testosterone injected into muscle would be absorbed and cleared almost immediately. Esterification changes that: the molecule sits in an oil depot and must be cleaved before the active hormone is available.

The length of the attached chain sets the pace. A short chain releases quickly; a longer one lingers. Everything about injectable testosterone since 1935 is a choice along that axis, trading injection frequency against stability of the resulting concentration.

The trade nobody escaped

Longer esters reduced the injection burden, and the field kept reaching for them. But the underlying criticism applied across the class. As one 1995 comparison put it, all injectable testosterone esters used clinically for substitution of male hypogonadism are characterised by unfavourable pharmacokinetics, and preparations producing constant physiological serum levels are what long-term treatment actually wants.[2]

That paper's own experimental comparison was conducted in long-term orchidectomised cynomolgus monkeys, not in men[2] — worth knowing before treating its measurements as human data. Its framing of the general problem, though, is a statement about the clinical preparations of the era.

What the dose-response data show about interval dosing

The clearest human illustration comes from a study that gave five different weekly doses of testosterone enanthate and reported the nadir — the low point of each interval. Mean nadir concentrations were 253, 306, 542, 1,345 and 2,370 ng/dL across the 25, 50, 125, 300 and 600 mg groups.[3]

Two things follow. There is a low point, which is what "interval dosing" means. And the dose determines where it sits — so raising the dose to lift the trough also raises the peak, and the study found hemoglobin rising and HDL cholesterol falling as concentration climbed.[3]

That trade is explored in what higher doses actually buy you.

Why propionate faded from maintenance use

Not because it failed. Because the frequency was impractical for long-term therapy, and longer esters existed. A man managing a chronic condition for decades will not sustain a several-times-weekly intramuscular injection if a weekly or quarterly option achieves the same target.

The guideline's framing captures why this matters clinically: choosing a formulation means weighing pharmacokinetics together with patient preference, formulation-specific adverse effects, treatment burden and cost.[4] Treatment burden is on that list for a reason, and compliance is one of the four things clinicians are asked to monitor.[4]

Frequently asked questions

Is propionate still used? It is not the standard for long-term replacement, where longer-acting preparations dominate. The reason is scheduling, not safety.

Does a shorter ester give more stable levels? More frequent dosing narrows the swing between doses, but each individual injection also clears faster. The pharmacokinetic argument for shorter intervals is set out — with its evidentiary limits — in daily vs weekly dosing.

Why not just inject plain testosterone? It would be cleared too quickly to be useful. The ester exists to slow release from the depot.

Next in this series

The ester that replaced it dominated for half a century.

Continue with enanthate, king for fifty years.

References

4 sources

1. Nieschlag E, Nieschlag S. ENDOCRINE HISTORY: The history of discovery, synthesis and development of testosterone for clinical use. Eur J Endocrinol. 2019;180(6):R201–R212. doi:10.1530/EJE-19-0071 · PMID 30959485
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. 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
4. 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