The ester is a fatty-acid tail attached to testosterone that controls how fast it leaves the oil depot. Short ester (propionate): fast in, fast out, injections every day or two. Medium (enanthate, cypionate): roughly a week of action, weekly or split injections. Long (undecanoate): months. The ester also takes up weight, so 250 mg of enanthate delivers about 180 mg of testosterone. Flat blood levels come from matching injection frequency to the ester, not from the ester itself.
- Once the ester is cleaved in the body, what circulates is testosterone. Esters differ in timing and weight, not in effect.
- Approximate half-lives: propionate under a day, enanthate and cypionate about 4–8 days, undecanoate in oil around 5–9 weeks.
- Ester weight: propionate is ~84% testosterone by mass, enanthate ~70%, cypionate ~69%, undecanoate ~63%.
- Enanthate and cypionate are pharmacologically interchangeable; the difference is folklore and oil solubility, not physiology.
- Peak-to-trough swing depends on injection interval relative to half-life: injecting a weekly ester once a week produces roughly a 2:1 swing; splitting it halves that.
What an ester does
Raw testosterone injected in oil is gone within hours. Attaching a fatty-acid chain (an ester) to the 17-beta position makes the molecule more oil-soluble, so it sits in the depot and leaks into the blood as enzymes slowly cleave the chain. Longer chain, slower leak. Once cleaved, it is plain testosterone. The ester decides timing and dose weight; it does not change what the hormone does.
The common esters
| Ester | Approx. half-life | Testosterone by weight | Typical clinical interval | Character |
|---|---|---|---|---|
| Propionate | ~0.8 day | ~84% | every 1–2 days | fast on, fast off, more injections, more site soreness |
| Phenylpropionate | ~1.5 days | ~66% | every 2–3 days | mostly in blends |
| Enanthate | ~4–5 days (clinical), up to ~8 reported | ~70% | weekly, often split | the reference ester |
| Cypionate | ~5–8 days | ~69% | weekly, often split | interchangeable with enanthate |
| Undecanoate (oil, IM) | ~5–9 weeks | ~63% | every 10–14 weeks after loading | few injections, slow to start and to leave |
Half-life figures vary between studies because they depend on oil, volume, injection depth and the person. Use them as ratios, not constants.
Ester weight: the number nobody prints on the label
The label says 250 mg per mL. That is the ester compound, not testosterone. Subtract the tail:
- Propionate 250 mg → about 210 mg testosterone
- Enanthate 250 mg → about 175–180 mg
- Cypionate 250 mg → about 172 mg
- Undecanoate 250 mg → about 158 mg
This matters when comparing products or reading studies: a trial quoting “600 mg enanthate” delivered about 420 mg of actual hormone. It also explains why switching from a short to a long ester at the same label dose feels weaker.
Frequency decides how flat your levels are
The swing between peak and trough depends on how often you inject relative to the half-life. For a weekly ester:
| Injection interval | Peak-to-trough ratio (approx.) |
|---|---|
| Every 7 days | ~2:1 |
| Every 3.5 days | ~1.4:1 |
| Every 2 days | ~1.2:1 |
| Daily (subcutaneous) | ~1.1:1 |
A 2:1 swing means estradiol, water, mood and blood pressure swing with it, and the peak is what pushes hematocrit and aromatisation. Splitting the same weekly amount removes most of that without changing the total. Microdosing takes this to its conclusion.
Steady state and when to test
Levels stabilise after about five half-lives: days for propionate, four to six weeks for enanthate or cypionate, many months for undecanoate. Blood drawn before that reflects the ramp, not the regimen. For the trough value that most labs and studies use, draw immediately before the next injection.
Choosing
- Most data, most predictable: enanthate or cypionate, split at least twice weekly.
- Fast in, fast out (for example, wanting clearance within days): propionate, at the cost of daily or every-other-day injections and more site irritation.
- Fewest injections: undecanoate, accepting a slow start, slow exit and less control.
- Blends of several esters give a fast initial rise plus a tail; they do not give flatter levels than a single ester injected at the right frequency.
Injection route also changes the curve: subcutaneous depots release more evenly than intramuscular ones, which is its own article.
Frequently asked questions
Which ester is best?
There is no best, only fit for purpose. Medium esters (enanthate, cypionate) are the workhorses: weekly action, steady levels when split, the most clinical data. Propionate suits people who want fast onset and fast clearance and accept frequent injections. Undecanoate suits people who want few injections and can tolerate a slow start.
Is enanthate stronger than cypionate?
No. Cypionate has one more carbon and slightly more ester weight, so a milligram of cypionate carries a fraction less testosterone. Half-lives overlap. Every comparison in the literature treats them as equivalent.
How much testosterone is in 250 mg of enanthate?
About 175–180 mg. The rest is the ester tail, which is cleaved and discarded. For undecanoate 250 mg gives about 158 mg; for propionate about 210 mg.
How often should a weekly ester be injected?
Once a week gives roughly a two-to-one swing between peak and trough with enanthate or cypionate. Twice a week flattens that to about 1.4:1, and more frequent or daily subcutaneous injections flatten it further. Flatter levels mean fewer estrogen swings and lower peaks driving hematocrit and blood pressure.
How long until levels are stable after switching esters?
About five half-lives to reach steady state: a few days for propionate, four to six weeks for enanthate or cypionate, many months for undecanoate. Bloodwork before steady state reflects the transition, not the new regimen.
Sources
- Kicman AT. Pharmacology of anabolic steroids. British Journal of Pharmacology, 2008.
- Bhasin S et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline (formulations table). JCEM, 2018.
- Serum testosterone concentrations remain stable between injections in patients receiving subcutaneous testosterone. Journal of the Endocrine Society, 2017.