If you've started reading about TRT, you'll have met a list of names that all seem to be testosterone and all seem to be different. Enanthate. Cypionate. Sustanon 250. Nebido. Forums argue about which is 'best' with a confidence the evidence doesn't really support, and the answers rarely mention the thing that matters most in the UK, which is what is actually licensed here.
This article explains what these names mean, how the injectable forms used in UK TRT differ from each other, where each sits legally, and why the choice between them is a prescribing decision rather than a shopping one.
A note on regulation before we start. UK law restricts the advertising of prescription-only medicines to the public, so no clinic should be promoting a specific branded product to you, and this article doesn't. It describes the options so that the conversation with your doctor makes sense. Which one you receive, if any, is their decision, made on your bloods and your circumstances.
What a testosterone ester is
Testosterone on its own is cleared from the body in about an hour. Injected as the raw hormone it would be gone almost before it started working, which is useless for a treatment that needs to hold levels steady for days or weeks.
The solution, worked out decades ago, is to attach a fatty acid chain to the testosterone molecule. That chain is the ester. The combination dissolves in oil, and once it is injected into tissue it sits there as a depot and seeps slowly into the bloodstream. As it enters circulation, enzymes in the blood snip the ester off and release ordinary testosterone, identical to the hormone your body makes.
So every ester delivers the same hormone. What differs is the length of the chain. Longer chains are more fat-soluble, leave the depot more slowly and keep working for longer. Shorter chains release faster and run out sooner. That single property, release speed, is the whole difference between the names on the list.
The esters used in UK TRT
| Ester | Duration | UK licensing | Typical use |
|---|---|---|---|
| Testosterone enantate (enanthate) | Medium-acting; measured in days | Licensed (Testosterone Enantate 250 mg/ml, Alliance Pharmaceuticals) | NHS and private; the most common ester in private UK TRT |
| Testosterone cypionate | Medium-acting; very similar to enantate | Not licensed in the UK; supplied as an unlicensed import on a prescriber's order | Some private clinics; widely used in the United States |
| Sustanon 250 (blend of four esters) | Mixed; short and long esters together | Licensed (Aspen) | NHS and private |
| Testosterone undecanoate (Nebido and generics) | Long-acting; measured in weeks to months | Licensed (Nebido, Grunenthal, plus generics) | The standard NHS injection; given in clinic |
| Testosterone propionate | Short-acting | Not available as a standalone UK-licensed injection; present as one component of Sustanon | Rarely used alone |
The duration column is deliberately vague. Exact half-lives vary with the oil the ester is dissolved in, the individual, and how the figure is measured, and the numbers quoted online contradict each other. What matters clinically is the ranking: propionate is shortest, enantate and cypionate sit in the middle and behave almost identically, and undecanoate is in a different league altogether.
Testosterone enantate
Spelt 'enantate' in UK medicine and 'enanthate' almost everywhere else, this is the workhorse of private TRT in the UK. It is licensed here for testosterone replacement in male hypogonadism, it is given intramuscularly under its licence, and it has a long clinical track record. Its medium duration is the useful property: long enough that a man isn't injecting every day, short enough that smaller, more frequent injections can hold levels steady and be adjusted precisely against blood results.
Testosterone cypionate
Cypionate is the American equivalent. It has a slightly longer ester chain than enantate, which makes it marginally longer-acting, but in practice the two are used interchangeably and the difference is not something you would notice.
The important point for a UK reader is that cypionate has no UK marketing authorisation. It does not appear in the electronic Medicines Compendium, which lists every licensed UK product. When it is used here, it is as an unlicensed medicine imported under the Human Medicines Regulations 2012, which require the importer to notify the MHRA before bringing it in. MHRA records released under Freedom of Information show several cypionate products, from manufacturers including Hikma and Hospira, notified for import on exactly that basis.
Unlicensed does not mean illegal or unsafe. The 'specials' regime exists precisely so that a doctor can prescribe something for an individual patient when a licensed product doesn't meet their needs, and it is used across medicine every day. But it does mean a prescriber has to justify the choice, and it means a clinic promoting cypionate as a selling point is on thin ground. If you are offered it, it is reasonable to ask why a licensed alternative wasn't suitable.
Sustanon 250
Sustanon is not a single ester. It contains four: testosterone propionate, phenylpropionate, isocaproate and decanoate, each with a different duration of action. The idea behind the blend, when it was designed, was that the short esters would get levels up quickly while the long ones kept them there, giving a more even release from a single injection given every few weeks.
It is licensed in the UK for male hypogonadism, it has been used for a very long time, and plenty of men are well on it. Its drawback is the flip side of its design. With four release curves overlapping, levels are harder to predict and harder to fine-tune than with a single ester, which is why many prescribers who want tight control prefer enantate. The 'four esters' marketing has also given Sustanon a mystique in gym culture that it doesn't deserve. It is testosterone, released over a mixed timeframe, and nothing more exotic than that.
Testosterone undecanoate (Nebido)
Undecanoate has a very long ester chain, and the injectable form is dissolved in castor oil, which slows release further still. The product information describes a depot that releases with a half-life measured in months, and a maintenance injection given every ten to fourteen weeks. One injection a quarter, roughly, which is why it is the standard NHS option: convenient for the patient and cheap in nursing time.
It is also the one you don't inject yourself. The licence requires a deep injection into the gluteal muscle, given very slowly over about two minutes, with the patient observed during and immediately afterwards because of a rare reaction in which droplets of oil reach the lungs and cause coughing or breathlessness. Hence a nurse, a clinic room, and an appointment every few months.
The trade-off, as our guide to testosterone injections explains, is the shape of the curve. There is a pronounced peak in the days after each injection and a trough in the final weeks before the next. Some men are entirely comfortable with that. Others describe the last fortnight of each cycle as a return to how they felt before treatment started, and it is the single most common reason men come to a private clinic from NHS treatment that is technically working.
Why private clinics favour the shorter esters
It comes down to control. Every injection produces a peak and a trough. The longer the gap between injections, the bigger the swing. Smaller, more frequent injections of a medium-acting ester such as enantate flatten the curve, so mood and energy stay steadier and the dose can be adjusted in small steps against your blood results. That is what most private UK clinics, including Arc, do, and it is the basis of the weekly or twice-weekly schedules you will read about.
One point of honesty here. The licensed dosing schedules for enantate and Sustanon describe injections some weeks apart. Giving smaller amounts more often than the licence describes is off-licence, but it is a legitimate and widespread prescribing decision, supported by multiple clinical studies showing steadier levels with smaller, more frequent doses. It is still a decision a doctor has to make and take responsibility for. It is another reason the schedule is set in a consultation, not copied from a forum.
None of this makes undecanoate wrong. For a man who is stable on it, has no trough symptoms and values four appointments a year over a weekly routine, it is a perfectly good treatment. The point is that the choice should follow your response and your life, not the other way round.
What the ester does not change
Whichever ester you end up on, the hormone it delivers is the same, and so is everything that comes with it. The monitoring is identical: full blood count for haematocrit, oestradiol, blood pressure, PSA where indicated, at baseline, around six to eight weeks in, then every three to six months. The side effects are the same, and so is their management. The effect on fertility is the same. Where the injection goes and how it is taught matters just as much; there is more on that in our guide to where testosterone is injected.
The ester changes the shape of your levels between doses. It does not change what testosterone is or what it does.
Whose decision it is
Yours to understand, your doctor's to make. A good prescriber will look at your blood results, your symptoms, how often you can realistically inject, whether you want to self-inject at all, your fertility plans, and what is licensed and available, and then recommend an ester and a schedule. They will also change it if your follow-up bloods say so. What they will not do is let you order from a menu, and a clinic that lets you is telling you how little clinical judgement is involved.
If you want to know what treatment at Arc includes and costs, with the medication, monitoring and reviews itemised, it is all on our pricing page. The product you end up on is settled with your doctor once your results are in.
Thinking about TRT?
Arc TRT is a UK doctor-led testosterone clinic that has helped over 3,000 men investigate their symptoms and, where appropriate, begin properly monitored treatment. Start with a simple at-home blood test.
This article is for general information and does not replace personalised medical advice. Testosterone is a prescription-only medicine and the choice of product is made by your prescribing clinician. Speak to a qualified clinician about your individual circumstances.
Sources: Summaries of Product Characteristics for Testosterone Enantate 250 mg/ml Solution for Injection (Alliance Pharmaceuticals), Sustanon 250 (Aspen) and Nebido 1000 mg/4 ml (Grunenthal), electronic Medicines Compendium, medicines.org.uk; electronic Medicines Compendium search of UK-licensed testosterone products; MHRA Freedom of Information response FOI 23/659, list of unlicensed medicines notified for import under Regulation 167 of the Human Medicines Regulations 2012; MHRA guidance, The supply of unlicensed medicinal products ('specials').
Keep reading
For the complete picture, see our full guide to TRT in the UK, covering eligibility, blood test thresholds, NHS and private routes, costs and monitoring. For the legal position on testosterone in the UK, see the legal section of that guide, and for what to expect once you start, read how TRT works and when you'll feel it. To compare injections with gels and tablets on frequency, steadiness and convenience, use our treatment options comparison.
