Testosterone Undecanoate

Testosterone undecanoate is a long-acting form of testosterone, most often given as a deep injection into a muscle every few months. It is common in the UK, Europe and Australia, where the injection is sold as Nebido or Reandron. An oral capsule with the same name exists but is used much less.

Where you'll meet it

On masculinizing prescriptions outside the US, often as the injection a GP practice nurse gives under a shared care arrangement with a gender clinic. In the US, shorter-acting injections such as testosterone cypionate and enanthate are far more usual; a US version of the long-acting injection exists but is sold under restricted conditions, so it is rarely used for trans men.

What the name means

Testosterone itself is cleared from the body quickly. Drug makers attach a chemical tail, called an ester, that has to be cut off before the hormone becomes active. Cypionate and enanthate are short tails; undecanoate is a long one. Dissolved in oil and injected into muscle, the long tail makes the testosterone release slowly over weeks. The hormone that ends up in your blood is the same testosterone either way.

What it means for you

The main difference is rhythm. Far fewer injections than the weekly or fortnightly short-acting ones, and levels that swing much less between peak and trough. Many people like not doing anything at home and not feeling an end-of-cycle dip.

The trade-offs:

• The injection is larger and oily, given slowly into the buttock, and some people find it painful.
• It has to be given by a clinician, so it depends on getting an appointment on time.
• Early on, the second dose usually comes sooner than later ones, while levels build up to a steady state.
• Adjusting takes a while. If your level runs high or low, it can take months to settle after a change, and a missed appointment has a long tail.

Your prescriber sets the interval and adjusts it to your blood results. Testosterone compares it with gels, short-acting injections and pellets.

How it is monitored

Blood tests are usually timed just before the next injection, at the trough, so the result shows whether the level is holding up through the whole interval. The aim is the same as for any testosterone: roughly the typical adult male range, often given as around 400 to 700 ng/dL (about 14 to 24 nmol/L). Hematocrit is watched as on any form; prescribers commonly start watching around 50 per cent and act somewhere around 50 to 54 per cent. How to read your hormone blood results covers units and timing.

A reaction to know about

Because it is an oily injection, a small number of people have a short reaction during or just after it: an urge to cough, breathlessness, chest tightness or feeling faint, thought to be caused by tiny droplets of oil reaching the lungs. It is usually brief and settles on its own, but it is why the injection is given slowly and why you are asked to wait in the clinic for a while afterwards. Allergic reactions are rarer still. Tell the nurse straight away if you feel any of this, and mention it before your next dose.

The oral version

Testosterone undecanoate capsules are absorbed through the gut's lymph system and need to be taken with food. Levels vary a lot across the day, and they are uncommon in gender-affirming care.

What to ask

Ask how the interval will be set and when bloods are due, what happens if an appointment slips, and whether you can switch to or from it later if your levels or routine change.

← Back to the Glossary