Esters and release rates

Why esterification changes a hormone's half-life from minutes to weeks, how chain length maps to release rate, and what that means for the shape of the DoseCurve chart.

Pure unesterified testosterone has a plasma half-life of roughly ten minutes. Nobody injects it. The reason injectable hormones are practical at all is esterification — attaching a fatty acid chain to the molecule, which makes it oil-soluble and turns the injection site into a slow-release depot. The longer the chain, the slower the release, the longer the apparent half-life, and the flatter the DoseCurve chart.

This page explains the mechanism, gives the half-life values used for common testosterone esters in the DoseCurve presets, and walks through the practical consequences for chart shape.

As elsewhere on the site: this is educational background, not a recommendation to use any specific compound at any specific dose. Talk to a clinician before starting, changing or stopping any prescribed medication.

What esterification actually does

Esterification is a chemistry trick. A hydroxyl group on the steroid is reacted with a carboxylic acid (the "ester") to form a new bond. The resulting molecule is markedly more fat-soluble than the parent, which has three consequences:

It can be dissolved in carrier oil at high concentrations, so a useful dose fits in a reasonable volume.
Once injected into muscle or subcutaneous tissue, it partitions into the surrounding fat-rich tissue and forms a depot.
The depot releases the esterified molecule slowly back into circulation, where serum esterases cleave the ester bond to release the active parent hormone.

The rate-limiting step is the slow release from the depot. The cleavage step (esterase activity in plasma) is fast and rarely the bottleneck. So the apparent serum half-life of an injected ester is governed mostly by how lipophilic the ester is — i.e., how tightly the depot holds onto it.

Chain length and half-life

Longer carbon chains mean greater lipophilicity, which means slower release. For testosterone esters specifically, the half-lives DoseCurve uses as presets (drawn from published pharmacology references) cluster around:

Ester	Carbon chain	Approx. half-life	Typical frequency in published literature
Testosterone propionate	C3	~0.8 days	Every 2–3 days
Testosterone phenylpropionate	aromatic	~1.5 days	Twice weekly
Testosterone enanthate	C7	~4.5 days	Weekly to every two weeks
Testosterone cypionate	C8 (cyclopentyl)	~8 days	Weekly to every two weeks
Testosterone undecanoate (oil)	C11	~21 days	Every 10–14 weeks (per Nebido SPC)

These values are population averages, and individual half-lives can vary substantially based on body composition, depot site, injection technique and depot volume. They are useful for predicting relative shape, not for setting absolute serum levels in any specific person.

What chain length does to the curve

Plot the same weekly dose at different half-lives and the differences become visible immediately:

Short ester (propionate, half-life ~0.8 days). Each injection produces a sharp peak that drops by half each day. Even a twice-weekly schedule shows visible peaks and troughs. The chart looks like a sequence of teeth.
Medium ester (enanthate, half-life ~4.5 days). A once-weekly schedule produces noticeable but rounded ripples around the steady-state average. Twice-weekly flattens those ripples significantly.
Long ester (cypionate, half-life ~8 days). Once-weekly produces a slightly wavy line near steady state. Twice-weekly is almost flat.
Very long ester (undecanoate, half-life ~21 days). The dose stacks for months. Once steady state is reached, the chart is almost imperceptibly above flat — but reaching that steady state takes the better part of a year.

For visual demonstration, set up two profiles in DoseCurve with the same weekly milligrams and different half-lives and the contrast is obvious within the first chart.

Why frequency matters more for short esters

The peak-to-trough ratio of a steady-state schedule is set by the dose interval relative to the half-life. As a rule of thumb, if the dose interval is much shorter than the half-life, the ratio approaches 1 (flat line). If the interval is around the half-life, the trough is about half the peak. If the interval is several half-lives, each dose largely clears before the next, and the ratio approaches the single-dose ratio.

That is why short esters effectively require frequent injections to maintain stable levels — and why long esters can be injected weekly or less without losing stability. It is not a clinical recommendation, just the mathematical consequence of the half-life.

Other compound classes

The same physics applies to other classes of injectables:

Nandrolone decanoate (~6 day half-life) and phenylpropionate (~2 day) follow the same pattern as the testosterone esters.
Trenbolone acetate (~1 day), enanthate (~5 day) and hexahydrobenzylcarbonate (~8 day) span a similar range.
GLP-1 analogues (semaglutide ~7 days, tirzepatide ~5 days) are not ester depots but reach long half-lives through albumin binding and structural modification. The kinetics still look first-order on a once-weekly schedule.
Peptides vary enormously: native peptides often have half-lives of minutes (ipamorelin, sermorelin), while modified peptides (CJC-1295 with DAC, ~8 days) can be very long-acting through albumin binding.

The DoseCurve substance presets give a starting half-life for each. They are overridable — if you have a published value that better fits your situation or that of the literature you are reading, change it.

Practical takeaways for reading the chart

Two compounds with the same weekly total dose can produce wildly different chart shapes. The shape is the half-life, not the dose.
Switching ester at the same total dose is a frequency conversation, not a dose conversation.
A long ester takes a long time to reach steady state. The chart's "time to steady state" metric will tell you how many days.
Quoted half-lives are population averages; bloodwork is the only way to anchor them in your own physiology.