Nebido (Testosterone Undecanoate): Complete UK Guide

Nebido is the brand name for testosterone undecanoate, a long-acting injectable testosterone preparation used to treat confirmed testosterone deficiency in adult men. Its defining feature is its duration of action: a single Nebido injection can maintain testosterone levels within the physiological range for 10 to 14 weeks, making it the least frequent injectable TRT option available in the UK.

For men who want the benefits of testosterone replacement therapy without the commitment of weekly or fortnightly injections, Nebido offers an appealing alternative. But it is not without trade-offs. This guide covers everything you need to know about Nebido testosterone undecanoate — from how it works and what the injection involves, through to costs, monitoring requirements, and how it compares to other TRT formulations available in the UK.

What Is Nebido?

Nebido is manufactured by Bayer and contains 1000 mg of testosterone undecanoate in 4 ml of castor oil. Testosterone undecanoate is a long-chain ester of testosterone, which means the testosterone molecule is attached to an undecanoic acid chain. This ester slows the release of testosterone from the injection depot significantly, resulting in a much longer duration of action compared to shorter-ester preparations like Sustanon 250 or testosterone enanthate.

The medication is classified as a prescription-only medicine (POM) in the UK and is licensed for the treatment of male hypogonadism when testosterone deficiency has been confirmed by clinical features and biochemical tests.

How testosterone undecanoate works

When Nebido is injected intramuscularly, the castor oil carrier creates a depot within the muscle tissue. The testosterone undecanoate is then slowly released from this depot over a period of weeks. As the ester bond is cleaved by esterase enzymes in the bloodstream, free testosterone is released and becomes biologically active.

The undecanoate ester has one of the longest half-lives of any testosterone preparation — approximately 33.9 days (plus or minus 4.9 days). This is what allows the 10 to 14-week injection interval that distinguishes Nebido from other injectable options.

After injection, testosterone levels typically rise over the first one to two weeks, reaching a peak before gradually declining over the following weeks. When the dosing interval is correctly calibrated, trough testosterone levels (measured just before the next injection is due) should remain within the normal physiological range.

Dosing Schedule for Nebido

The standard dosing protocol for Nebido follows a specific loading and maintenance pattern:

Phase	Timing	Dose	Notes
First injection	Day 0	1000 mg (4 ml)	Baseline blood tests should be completed before this
Second injection (loading dose)	6 weeks after first	1000 mg (4 ml)	Shorter interval to establish therapeutic levels
Maintenance injections	Every 10–14 weeks	1000 mg (4 ml)	Interval adjusted based on trough blood levels and symptoms

The initial loading phase is important because testosterone undecanoate takes time to accumulate to steady-state levels. The six-week gap between the first and second injection helps establish a therapeutic baseline more quickly than a standard 10 to 14-week interval would.

Adjusting the injection interval

The prescribing information allows for a maintenance interval of 10 to 14 weeks, and finding the right interval for each patient is a key part of clinical management. This is determined by measuring trough testosterone levels — a blood sample taken just before the next injection is due.

• If trough testosterone is below the reference range, the interval should be shortened (for example, from 12 weeks to 10 weeks)
• If trough testosterone is above the upper end of the reference range, the interval may be extended
• Most men find their optimal interval falls between 10 and 12 weeks, though some require the full 14-week interval

Symptom tracking is equally important. Some men report a return of low testosterone symptoms (fatigue, low mood, reduced libido) in the final one to two weeks before their next injection, even when blood levels are technically within range. In these cases, shortening the interval by one or two weeks can make a meaningful difference to quality of life.

How Is Nebido Administered?

Nebido is administered as a deep intramuscular injection, typically into the gluteal muscle (the upper outer quadrant of the buttock). The injection must be given very slowly, over a period of approximately two minutes, due to the large volume (4 ml) and viscosity of the castor oil carrier.

Can you self-inject Nebido?

Unlike shorter-acting testosterone preparations such as Sustanon 250 or testosterone enanthate, Nebido is most commonly administered by a healthcare professional in a clinical setting. There are several reasons for this:

• Volume: Injecting 4 ml of viscous oil requires a slow, controlled technique that can be difficult to manage when self-injecting into the gluteal muscle
• Injection site: The gluteal muscle is the recommended site, which is harder to reach independently compared to the thigh
• Post-injection observation: The SmPC for Nebido recommends a brief observation period after injection due to the rare risk of pulmonary oil microembolism (POME) — a reaction that can cause cough, shortness of breath, or chest tightness immediately after injection
• Clinical preference: Many prescribers prefer to administer Nebido themselves to ensure correct technique and placement

That said, some experienced TRT patients are trained to self-inject Nebido at home. This is an off-label approach but is supported by some specialist clinicians, particularly for patients who have tolerated several clinic-administered injections without adverse reactions and who demonstrate competent injection technique.

What does the injection feel like?

Nebido injections are generally more noticeable than smaller-volume testosterone injections. The 4 ml volume means the injection takes longer, and the viscous oil creates a sensation of pressure in the muscle as it is deposited. Most men describe the experience as uncomfortable rather than painful, though individual tolerance varies.

Post-injection soreness is common and can last for one to three days. Some men develop a firm lump at the injection site that takes several days to fully disperse. Alternating between left and right gluteal muscles with each injection helps minimise cumulative tissue irritation.

Advantages of Nebido

Nebido has several distinctive advantages that make it the preferred TRT option for certain patients:

• Infrequent injections: Four to five injections per year is the main selling point of Nebido. For men who dislike needles, have busy schedules, or travel frequently, this reduced injection burden is a significant benefit
• Relatively stable blood levels: While not as stable as weekly or twice-weekly short-ester injections, Nebido generally produces smoother blood level curves than Sustanon 250 injected every three weeks. The long half-life avoids the sharp peaks seen with shorter-acting preparations
• No peanut oil: Nebido uses castor oil as its carrier, making it suitable for men with peanut or soya allergies who cannot use Sustanon 250 (which contains arachis oil)
• Reduced oestrogen spikes: The more gradual testosterone release means less dramatic aromatisation (conversion to oestrogen) compared to the sharp peaks produced by shorter-acting preparations
• Simplified routine: For men who struggle with the discipline of regular self-injection, having a scheduled clinic appointment every 10 to 14 weeks removes the risk of missed or delayed doses

Disadvantages of Nebido

Despite its convenience, Nebido is not the ideal choice for everyone:

• Large injection volume: The 4 ml injection is significantly larger than the 0.5 to 1 ml volumes used for other testosterone preparations, which can cause more discomfort and post-injection soreness
• Limited self-injection: Most men need to attend a clinic for each injection, which adds logistical complexity and potentially travel time
• Higher per-injection cost: Nebido is substantially more expensive per injection than Sustanon or enanthate (see cost section below)
• Less dosing flexibility: Because the entire 1000 mg dose is administered at once, fine-tuning dosage is more difficult than with shorter-acting preparations where weekly doses can be adjusted in small increments
• End-of-cycle dip: Some men experience a noticeable decline in wellbeing in the final one to three weeks before their next injection. While generally less severe than the troughs seen with infrequent Sustanon dosing, this can be frustrating
• Risk of POME: Although rare, pulmonary oil microembolism is a specific risk associated with Nebido that does not apply to smaller-volume oil-based injections. Symptoms include sudden cough, difficulty breathing, chest pain, and dizziness. This is one reason post-injection observation is recommended
• Long washout if problems arise: If you experience significant side effects, the long duration of action means the medication cannot be quickly cleared from your system. With shorter-acting preparations, stopping injections results in a more rapid return to baseline

Side Effects of Nebido

The side effects of Nebido are broadly similar to those of other testosterone replacement therapies, as they relate to the testosterone itself rather than the specific ester or carrier. However, there are some considerations specific to this formulation.

Common side effects (affecting more than 1 in 10 men)

• Injection site reactions (pain, redness, firmness, or swelling)
• Polycythaemia (elevated red blood cell count / haematocrit)

Common side effects (affecting 1 in 10 to 1 in 100 men)

• Acne and oily skin
• Weight gain
• Increased PSA (prostate-specific antigen)
• Mood changes
• Increased haemoglobin

Uncommon but important side effects

• Gynaecomastia (breast tissue growth) due to oestrogen conversion
• Sleep apnoea (may worsen existing condition)
• Changes in liver function markers
• Lipid profile changes (reduction in HDL cholesterol)
• Increased blood pressure
• Pulmonary oil microembolism (POME) — rare but specific to oil-based depot injections

The most clinically significant side effect across all forms of TRT, including Nebido, is polycythaemia. The NICE guidelines and British Society for Sexual Medicine (BSSM) recommendations both emphasise the importance of regular haematocrit monitoring. If your haematocrit rises above 0.54 (54%), your clinician will need to take action — this may involve extending the injection interval, reducing the dose (where possible with other formulations), or recommending venesection (therapeutic blood removal).

Monitoring Requirements

Proper monitoring is essential for safe and effective Nebido therapy. At Evernu, we follow evidence-based monitoring protocols that include:

Before starting treatment

• Two morning blood samples confirming total testosterone below the lower reference range
• Full baseline blood panel: total testosterone, free testosterone, SHBG, LH, FSH, prolactin, oestradiol, full blood count, haematocrit, PSA, liver function, renal function, lipid profile, HbA1c, thyroid function
• Clinical assessment of symptoms and medical history
• Discussion of fertility considerations (TRT suppresses sperm production)

During treatment

Timepoint	Key tests
Before second injection (6 weeks)	Total testosterone, haematocrit, full blood count
Before third injection (approx. 16–20 weeks)	Comprehensive blood panel
6 months	Comprehensive blood panel including PSA and lipids
12 months	Full annual review with comprehensive blood panel
Ongoing	Trough blood tests before each injection, comprehensive panel every 6–12 months

Trough blood samples are particularly important with Nebido. Because the injection interval can be adjusted between 10 and 14 weeks, having accurate trough data allows your clinician to fine-tune the timing to keep your levels optimally within range.

If you are currently on Nebido but are not receiving regular monitoring, we would strongly recommend arranging a testosterone blood test to check your current levels and key safety markers.

How Much Does Nebido Cost in the UK?

Cost is a relevant consideration when choosing a TRT formulation, and Nebido is the most expensive injectable option per injection:

Access route	Approximate cost
NHS prescription	£9.90 per prescription (standard charge in England; free in Scotland, Wales, and Northern Ireland). The NHS cost of the medication itself is approximately £80–£90
Private prescription (medication only)	£80–£120 per injection
Private clinic (including administration)	£120–£200 per injection (includes nursing time for administration)

On an annual basis, assuming four to five injections per year, the medication cost for Nebido works out to approximately £320 to £600 per year privately, plus consultation and blood test costs. This compares to approximately £36 to £120 per year for Sustanon 250 (medication only) or £240 to £480 per year for testosterone enanthate.

However, the total cost comparison is not straightforward. Men on Nebido require fewer clinic visits for injection administration, and the per-injection monitoring bloods may also be less frequent (as they coincide with the injection schedule). For a detailed breakdown of TRT costs across all options, see our complete guide to TRT costs in the UK.

How Nebido Compares to Other TRT Options

Nebido vs Sustanon 250

The most common comparison, as both are widely available in the UK:

• Injection frequency: Nebido every 10–14 weeks vs Sustanon every 2–3 weeks (or weekly with split dosing)
• Self-injection: Sustanon is routinely self-injected at home; Nebido usually requires a clinic visit
• Cost: Sustanon is significantly cheaper per year, especially on the NHS
• Blood level stability: Both can produce stable levels when dosed optimally, though Sustanon requires more frequent injections to achieve this
• Allergy considerations: Men with peanut/soya allergies cannot use Sustanon but can use Nebido (castor oil carrier)

Nebido vs testosterone enanthate

• Injection frequency: Nebido every 10–14 weeks vs enanthate every 1–2 weeks
• Blood level stability: Weekly enanthate generally produces the most stable testosterone levels of any injectable option, but requires far more frequent injections
• Dose flexibility: Enanthate allows fine-grained dose adjustments (changing weekly dose by 10–25 mg increments), while Nebido dosing is less adjustable
• Self-injection: Enanthate is easily self-injected in small volumes; Nebido usually requires professional administration

Nebido vs testosterone gel

• Application: Gel is applied daily to the skin; Nebido is injected every 10–14 weeks
• Absorption consistency: Gel absorption can vary with skin conditions, sweating, and showering; Nebido absorption is more predictable
• Transfer risk: Gel carries a risk of transferring testosterone to partners or children through skin contact; Nebido has no transfer risk
• Convenience: Daily gel application vs quarterly injection — which is more convenient depends on the individual

Who Is Nebido Best Suited For?

Based on clinical experience, Nebido tends to work particularly well for:

• Men who strongly prefer infrequent injections and are willing to attend a clinic for administration
• Men with peanut or soya allergies who cannot tolerate Sustanon 250
• Men who have difficulty maintaining a regular self-injection schedule
• Men who travel frequently and find it impractical to carry injectable testosterone and supplies
• Men who have tried daily gel application and found it inconvenient or ineffective due to poor absorption

Nebido may be less suitable for:

• Men who want maximum control over their dosing and blood levels
• Men who are cost-sensitive (particularly those paying privately)
• Men who experience significant end-of-cycle symptoms despite interval adjustment
• Men who prefer to self-administer treatment at home
• Men of reproductive age who may want to preserve fertility options (all TRT suppresses sperm production, but the long duration of Nebido makes it harder to pause treatment for fertility purposes)

Frequently Asked Questions About Nebido

How long does it take for Nebido to start working?

After the first injection, most men begin to notice improvements in energy and mood within one to three weeks. However, because the loading phase involves only two injections over six weeks, it can take two to three months to reach stable therapeutic levels. Full benefits of treatment — including improvements in body composition, sexual function, and bone density — develop over three to twelve months, consistent with other forms of TRT.

Can I switch from Sustanon or enanthate to Nebido?

Yes, switching between testosterone formulations is straightforward and commonly done. Your clinician will advise on the timing of the switch — typically, the first Nebido injection is given at the point where your next Sustanon or enanthate injection would have been due. Blood levels should be checked before the second Nebido injection to confirm the new protocol is achieving therapeutic levels.

Does Nebido affect fertility?

Yes. Like all forms of exogenous testosterone, Nebido suppresses the hypothalamic-pituitary-gonadal (HPG) axis, which significantly reduces sperm production. This effect may or may not be fully reversible after stopping treatment. Men who wish to preserve fertility should discuss this with their clinician before starting any form of TRT, and alternative approaches such as hCG or selective oestrogen receptor modulators (SERMs) may be considered.

What happens if I miss a Nebido injection?

If your injection is delayed by a few days to a week, this is unlikely to cause significant problems. However, if the interval extends well beyond your normal schedule, testosterone levels may fall below the therapeutic range and symptoms of low testosterone may return. Contact your clinic to reschedule as soon as possible. There is no need to “double up” on the dose — simply resume the normal 1000 mg injection and recalibrate the interval.

Is Nebido available on the NHS?

Yes, Nebido is available on NHS prescription, though prescribing practices vary between regions and individual prescribers. Some NHS endocrinologists and GPs prescribe it routinely, while others prefer Sustanon 250 due to its lower cost. If you believe Nebido would be more suitable for you than your current NHS prescription, discuss this with your prescriber — a clinical rationale (such as peanut allergy or intolerance to frequent injections) can support the request.

Getting Started with Nebido at Evernu

If you are considering Nebido as part of your testosterone replacement therapy, or if you would like to explore whether TRT is appropriate for you, Evernu can help.

We offer comprehensive testosterone assessments that include thorough blood testing, clinical evaluation, and personalised treatment recommendations. Our clinicians are experienced in prescribing all forms of TRT, including Nebido, and we provide ongoing monitoring to ensure your treatment is safe and effective.

You can start by taking our free online testosterone assessment to evaluate your symptoms, or by ordering a testosterone blood test to check your current levels.

To learn more about our approach to testosterone replacement therapy, visit our TRT treatment page.

This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting or changing any medication. Evernu is regulated by the Regulation and Quality Improvement Authority (RQIA).