What Is Nandrolone Decanoate (Deca)? Half-Life & Dosing

Quick answer

Nandrolone decanoate is testosterone’s 19-nor cousin attached to the decanoate ester, a long oil-soluble chain that lets it be injected roughly weekly to biweekly instead of daily. The ester’s half-life runs ~6–12 days, and about 64% of the milligrams in the vial are the actual nandrolone. It barely aromatizes to estrogen but carries its own risk profile — most notably to lipids and prolactin — separate from testosterone’s. Educational only; any protocol and its monitoring belong with a prescriber.

What nandrolone decanoate is and how it works

Nandrolone is testosterone with one structural change — the removal of a carbon at the 19 position — which is why it and its relatives (NPP, trenbolone) are grouped as 19-nor steroids. That single change reshapes how the molecule is metabolized: instead of converting to the potent androgen DHT the way testosterone does, nandrolone is reduced to dihydronandrolone (DHN), a comparatively weak androgen. It also aromatizes to estrogen at only a fraction of testosterone’s rate.

The trade-off is that nandrolone binds the progesterone receptor meaningfully, which testosterone does not. That progestogenic activity is behind a chunk of nandrolone’s distinctive side-effect list — more on that below — and is a large part of why its risk profile reads differently from a testosterone-only picture even at doses that would otherwise look modest.

The decanoate ester is the long-acting option: an oil-soluble chain bonded to the nandrolone molecule that creates a depot at the injection site and releases slowly. About 64% of the milligrams in nandrolone decanoate are nandrolone itself; the rest is ester weight that comes along for the ride and is cleaved off before the hormone is active.

Dosing math & the mg/mL trap

Like testosterone, nandrolone decanoate is labeled by concentration — milligrams per milliliter — not total milligrams per vial. The same one-line formula converts a milligram amount into a syringe volume: volume (mL) = dose (mg) ÷ concentration (mg/mL).

Reference concentrations vary by source, which is exactly why running your own numbers through a calculator matters more than memorizing a table.

Amount	100 mg/mL	200 mg/mL	300 mg/mL
100 mg	1.00 mL (100u)	0.50 mL (50u)	0.33 mL (33u)
200 mg	2.00 mL (200u)	1.00 mL (100u)	0.67 mL (67u)
300 mg	3.00 mL (300u)	1.50 mL (150u)	1.00 mL (100u)
400 mg	4.00 mL (400u)	2.00 mL (200u)	1.33 mL (133u)

Volume to draw at three reference concentrations (U-100 syringe units in parentheses). Reference numbers for the arithmetic, not a recommended amount — the HRT tab of the calculator does the math for any concentration you actually have.

Half-life & what it means for frequency

A ~6–12-day ester half-life is long by injectable-steroid standards, which is why nandrolone decanoate is typically run weekly or every other week rather than more often. It also means it takes several weeks of consistent injections to reach a steady blood level — the depot has to build up before concentrations stop climbing week over week.

The other side of a long ester is a long tail after the last shot: nandrolone stays detectable and physiologically active for weeks to months after stopping. That matters for planning — if suppression, prolactin, or lipid changes show up, they do not resolve quickly just because the injections stopped.

What to monitor & the real risks

Nandrolone’s progestogenic activity is the thing that most distinguishes it from testosterone on a lab panel: prolactin is worth tracking, since elevated prolactin can drive gyno-like breast tissue changes and, in some, unwanted effects on libido and erectile function. Estradiol still matters too, even though nandrolone aromatizes less than testosterone.

The cardiovascular side is significant and often underweighted: nandrolone has one of the more pronounced effects on HDL cholesterol among AAS, and a full lipid panel before and during use is a reasonable baseline. Hematocrit is standard AAS monitoring generally — thicker blood raises clotting risk. And because it suppresses the HPTA like other AAS, total/free testosterone and LH/FSH are worth a baseline if fertility or post-use recovery is a concern; the long ester means that recovery timeline runs long too.

Frequently asked questions

What is the half-life of nandrolone decanoate (Deca)?

The decanoate ester’s half-life runs roughly 6–12 days, which is why it is typically injected weekly to every other week. That long ester also means it takes several weeks to reach a steady blood level and leaves a long tail after the last injection.

Why does Deca affect prolactin and libido?

Nandrolone binds the progesterone receptor more than testosterone does, and that progestogenic activity can raise prolactin in some users. Elevated prolactin is linked to gyno-like tissue changes and, for some, reduced libido or erectile difficulty — which is why prolactin is worth including in bloodwork alongside estradiol.

How do you calculate a nandrolone decanoate dose in mL?

Volume (mL) = dose (mg) ÷ concentration (mg/mL). A 200 mg amount from a 200 mg/mL vial is 1.00 mL, or 100 units on a U-100 syringe. The HRT tab of the calculator does this for any concentration.

What should I monitor on nandrolone decanoate?

A lipid panel (nandrolone is known for suppressing HDL), hematocrit, prolactin, estradiol, and total/free testosterone with LH/FSH if fertility matters. Because the ester is long, changes in any of these can take weeks to show up — and weeks to resolve after stopping.

This is educational information, not medical advice. Doses and protocols above reflect published references and community practice — protocol decisions belong with your prescribing physician. Generic names used throughout. How this reference is built & how the AI is tested →

What Is Nandrolone Decanoate? Half-Life, Dosing Math & How to Track It