One thing before anything else. This article explains the injection sites and routes used in UK testosterone therapy, and why your doctor might choose one over another. It is not a how-to. Injection technique is taught by your prescribing clinic, in person or on video, with your particular medication and your particular body in front of them. Nothing here replaces that, and nothing here is written to let you skip it.
With that said, 'where does the injection go?' is a fair question to ask before you start, and the answer tells you quite a lot about how modern TRT works.
The two routes
Testosterone injections in the UK are oil-based. The testosterone is attached to an ester and dissolved in oil, which is what allows it to release slowly over days or weeks rather than all at once. That oil has to be delivered into a tissue that will absorb it steadily, and there are two candidates: muscle or the fat layer just under the skin.
Intramuscular
The traditional route, and the one every UK-licensed testosterone injection is actually licensed for. The product information for Sustanon, testosterone enantate and testosterone undecanoate all say the same thing: inject deep into muscle, and inject slowly.
The muscles used are the ones big enough to take an oily injection comfortably:
- The buttock (gluteal muscle). The classic site for larger volumes. The long-acting undecanoate injection is given here in clinic, slowly, with the patient observed afterwards.
- The outer thigh (vastus lateralis). Easier to reach yourself than the buttock, which is why it's commonly taught for self-injection.
- The shoulder (deltoid). Suitable for small volumes only, and less often used for testosterone.
Intramuscular injection uses a longer needle, because it has to reach the muscle through the fat on top of it. That's part of why it has a reputation for being uncomfortable, and part of why the depth matters so much, which we'll come back to.
Subcutaneous
Subcutaneous injection goes into the fat layer under the skin, usually of the abdomen, with a short, fine needle of the kind used for insulin. It has become widely used in private TRT over the last decade for a simple reason: it is generally more comfortable, it is easier to do yourself, and for many men it achieves comparable blood levels to intramuscular injection.
The published evidence is encouraging but still fairly small, and much of it comes from transgender men rather than men with hypogonadism. A 2017 study of 63 patients found that weekly subcutaneous testosterone achieved normal male testosterone levels in every patient across a wide range of body weights, and that patients who had previously used intramuscular injections preferred it. A companion study found levels stayed stable between weekly injections. Larger head-to-head trials in hypogonadal men are still lacking.
One point your clinic should be open about: because the UK product licences specify intramuscular use, subcutaneous injection of these products is an off-licence use. That is legal, common and well supported: several clinical studies, including those in our sources, show subcutaneous injection producing normal, stable testosterone levels. It does mean the prescribing doctor takes responsibility for the decision, which is one more reason the route should be decided clinically rather than ticked on a sign-up form.
How the two compare
| Item | Intramuscular | Subcutaneous |
|---|---|---|
| Where | Buttock, outer thigh, occasionally shoulder | Fat layer of the abdomen |
| Needle | Longer, to reach muscle | Short, fine insulin-type needle |
| Comfort | Can be uncomfortable, especially the buttock | Generally more comfortable |
| Self-injection | Possible, usually taught on the thigh | Easier to learn and do yourself |
| Licensing | The licensed route for all UK testosterone injections | Off-licence; a prescriber's clinical decision |
| Suits everyone? | Most men | Most men, but some don't absorb well subcutaneously |
As our guide to testosterone injections puts it, the right route for you is a clinical decision based on how you actually respond, not a preference expressed at sign-up. Some men simply do not absorb testosterone well from the fat layer and get better, steadier levels from muscle. You find that out from follow-up bloods, not from a forum.
Why frequency matters more than site
Men tend to focus on where the needle goes. In practice, how often you inject has a far bigger effect on how you feel.
Every injection produces a peak in the days afterwards and a trough before the next one. The further apart the injections, the higher the peak and the deeper the trough, and that swing is what men experience as good weeks and bad weeks: energy and mood up after a dose, then fading. The long-acting undecanoate injection, given every ten to fourteen weeks, is the clearest example. Some men do well on it. Others describe the last fortnight of each cycle as a return to how they felt before treatment.
Smaller, more frequent injections of a shorter-acting ester flatten that curve. Levels stay closer to steady, mood and energy swing less, and the dose can be adjusted precisely against your blood results. That is why most private clinics, including Arc, favour weekly or twice-weekly injections. Which site you use for those injections matters far less than keeping to the schedule.
Why technique has to be taught, not guessed
Here is the problem with learning injection technique from the internet. If an intramuscular injection doesn't go deep enough, the oil ends up sitting in fat rather than muscle. If a subcutaneous injection goes in at the wrong angle or the wrong site, absorption changes. In both cases the testosterone still goes in, but it's absorbed differently from how your doctor expects, and the result on your next blood test looks like a dosing problem.
So the dose gets adjusted. The actual issue, which was technique, carries on. We see this often enough that it is a standard question when a man's levels don't make sense: show me exactly how you're injecting.
Proper teaching also covers the things that keep an injection safe rather than just effective: rotating sites so the same patch of tissue isn't used every time, recognising the difference between normal post-injection soreness and a reaction that needs a call to the clinic, and what to do if something doesn't look right. Your clinic should walk you through all of this before your first dose and be available afterwards. If a provider posts you a vial with a leaflet and nothing else, that tells you something about the provider.
Hygiene and sharps
Two practical points your clinic will cover, and which the NHS is clear about.
Needles and syringes are single use. A new sterile needle every time, skin cleaned beforehand, hands washed. None of this is complicated, but it is non-negotiable, and infections at injection sites are almost always a hygiene failure.
Used needles and syringes go into a proper sharps bin: a rigid, lidded container that your GP can prescribe or a pharmacy can sell you. Never a household bin, never the recycling, never a drinks bottle. Don't recap, bend or snap needles, don't fill the bin above the marked line, and once it's full seal it and arrange collection through your council or pharmacy, which varies by area. Your clinic should supply a sharps bin with your first prescription and explain the disposal arrangements where you live.
What monitoring comes with injections
Whichever route and schedule you end up on, the monitoring is the same, and it is not optional. Testosterone raises red blood cell production, so a full blood count for haematocrit is checked at baseline and at every review. Oestradiol is tracked, because some testosterone converts to oestrogen and both too much and too little cause problems. Blood pressure is monitored at home. PSA is checked where indicated. Follow-up bloods usually happen around six to eight weeks after starting, then every three to six months once stable.
There is a fuller account of what is common, what is rare and what to do about each in our guide to TRT side effects. The short version is that the needle is the easy part. The monitoring around it is what makes TRT safe.
If you're already injecting and it isn't going well
Some men reading this are already on treatment, with another clinic or on their own, and are getting inconsistent results, sore injection sites or levels that never seem to settle. Often the fix is a change of route, a change of frequency, or simply having someone watch you inject and correct one small thing. If your current provider isn't doing that, switching clinic is straightforward and doesn't mean starting from scratch.
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. Injection technique must be taught by your prescribing clinician. Speak to a qualified clinician about your individual circumstances.
Sources: Summaries of Product Characteristics for Sustanon 250, Testosterone Enantate 250 mg/ml and Nebido (testosterone undecanoate), electronic Medicines Compendium, medicines.org.uk; Spratt D.I. et al., Subcutaneous Injection of Testosterone Is an Effective and Preferred Alternative to Intramuscular Injection, Journal of Clinical Endocrinology and Metabolism, 2017; McFarland J. et al., Serum Testosterone Concentrations Remain Stable Between Injections in Patients Receiving Subcutaneous Testosterone, Journal of the Endocrine Society, 2017; Kent Community Health NHS Foundation Trust, Disposing of sharps safely; The Royal Marsden Manual of Clinical Nursing Procedures, intramuscular injection sites.
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 medications themselves, read our comparison of testosterone enanthate, cypionate and Sustanon, and for what the first months feel like, see how TRT works and when you'll feel it.
