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. In the US, shorter-acting injections such as testosterone cypionate and enanthate are more usual.

How it differs from other injections

"Undecanoate" is a long chemical tail that makes the testosterone release slowly. That means far fewer injections than the weekly or fortnightly shorter-acting ones, and levels that swing less between peak and trough. The trade-off is that the injection is larger, has to be given by a clinician, and early on the gap between doses is usually shorter while levels build up. Your prescriber sets the interval and adjusts it to your blood results.

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: coughing, breathlessness or feeling faint, caused by tiny droplets of oil reaching the lungs. It is usually brief, but it is why the injection is given slowly and why you are asked to wait a little while in the clinic afterwards. Testosterone compares the forms.

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