Testosterone replacement therapy is used to restore hormone levels in men when the body is not producing enough on its own. Low testosterone can cause fatigue, reduced sex drive, weight gain, and erection problems.1
Treatment helps bring levels back into a healthy range. TRT dose formats may include injections, gels, patches, or capsules, depending on what your provider has access to and your clinical needs. The aim is not to raise testosterone beyond normal levels but to correct an imbalance so you can feel more like yourself again.2
For more information, visit our 'What is TRT' page.
The side effects men are most likely to notice when starting TRT are listed below.3 For a complete list of side effects, please refer to the patient information leaflet found in the packaging your medication comes in.
| Acne | Injection site reactions | Increased red blood cell count |
|---|---|---|
| Increased haematocrit (a blood marker) | Thicker blood | Increased appetite |
| Weight gain | Hot flushes | Increased prostate-specific antigen (prostate marker) |
Some side effects from TRT are considered uncommon, meaning they affect between 1 in 100 and 1 in 1,000 people.3
Uncommon side effects of TRT include:3
Allergic reactions
Increased appetite
High cholesterol
Irritability
Headaches
If you experience any of these, it is important to let your healthcare provider know so they can monitor and support you.
There are also some rare side effects associated with certain forms of TRT. Rare means they affect between 1 in 1,000 and 1 in 10,000 people. One example is something called pulmonary oil microembolism.3
This happens when a tiny amount of the oily solution used for injections enters the bloodstream and reaches the lungs. It can cause sudden symptoms like cough, chest pain, dizziness, or shortness of breath during or just after the injection.4
Other rare side effects of TRT include:
Liver damage
Priapism
Sleep apnoea
Not everyone who takes TRT will experience side effects, and many are mild or temporary. Some symptoms, like acne or mild nausea, are self-limiting and often settle on their own as your body adjusts to treatment.
If side effects persist, your clinician may recommend adjusting your dose. This can help balance the benefits of TRT while minimising unwanted effects. In some cases, you might be advised to switch to a different formulation, such as moving from injections to gels or patches, to see if that suits you better.
Rarely, if side effects are more severe or cannot be managed safely, your clinician may recommend stopping TRT altogether. Any decision about changes to your treatment will always be guided by your healthcare provider, who will monitor your response with regular check-ins and blood tests.
The most important step is to keep open communication with your clinician. If you notice any new or concerning symptoms, let them know right away so you can work together on the best way forward.
The safest way to start TRT is through a regulated clinic, either in person or online, where treatment follows strict medical guidelines and national safety limits.
Avoid black market products or offers from unlicensed vendors, such as people in gyms or online forums. These are not only illegal but also risky, as the medicines may be counterfeit, unsafe, or dosed incorrectly. A credible healthcare provider will run proper blood tests, assess your symptoms, and create a plan that treats you as an individual, with ongoing monitoring to make sure the therapy is both safe and effective.
The side effects that come with TRT are often formulation-dependent. Below are the main differences in side effects between different TRT formulations.
Regular injections
These can cause hormone level “peaks and troughs,” which may lead to mood changes, tiredness, or fluctuations in symptoms. Injection site pain and, in rare cases, blood clots have also been reported.5
3-monthly injections
Most common side effects are acne and pain at the injection site. Rarely, a reaction called pulmonary oil microembolism can occur during or just after the injection, causing short-lived chest symptoms.6
Gels
Gels often cause skin irritation, redness, or acne at the application site. Care is needed to avoid accidental transfer of the medication to partners or children through skin contact.7
Capsules
Capsules are convenient but can raise blood pressure, alter cholesterol, and sometimes worsen sleep apnoea. They may also cause water retention, leading to swelling in the ankles or legs.8
Our men’s health team brings together years of experience from both the NHS and private practice. Having worked across leading healthcare organisations in the UK and internationally, they combine clinical expertise with the latest research to deliver evidence-based treatments. At Numan, they apply this knowledge to men’s health, tailoring care to each individual’s needs with the same commitment to excellence.
Dr Dimitris Schizas
Numan Doctor
MBBS MRCGP MSC BSSM
Dr Michael Lacey
Numan Doctor
MBChB BSc (Hons) MRCGP
Dr Alexandra Davidson
Numan Doctor
BSc (Hons) MBBS MRCGP AFHEA MA (Hons) AFMCP
Abby Watts
Senior Nurse Prescriber
BSc Adult Nursing, IP
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1. Sizar O, Leslie SW, Schwartz J. Male hypogonadism. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.
2. Barbonetti A, D’Andrea S, Francavilla S. Testosterone replacement therapy. Andrology. 2020;8(6):1551–66.
3. Testosterone replacement therapy : (TRT). Nhs.uk. [accessed 17 Feb 2026] Available from: https://www.ncic.nhs.uk/patients-visitors/patient-information-leaflets/Testosterone-replacement-therapy
4. Meyer RJ, Mann M. Pulmonary oil micro-embolism (POME) syndrome: a review and summary of a large case series. Curr Med Res Opin. 2015;31(4):837–41.
5. DEPO®-TESTOSTERONE (testosterone cypionate) Description. Pfizermedical.com. [accessed 25 Sept 2025] Available from: https://www.pfizermedical.com/depo-testosterone/description
6. Nebido 1000 mg/4 ml solution for injection. Org.uk. [accessed 25 Sept 2025] Available from: https://www.medicines.org.uk/emc/product/14631/smpc
7. TESTOGEL 50 mg, transdermal gel in sachet. Org.uk. [accessed 25 Sept 2025] Available from: https://www.medicines.org.uk/emc/product/6808/smpc
8. Oral Testosterone Replacement - KYZATREX® (testosterone undecanoate). Kyzatrex. 2024. [accessed 25 Sept 2025] Available from: https://www.kyzatrex.com/
Updated:
Dr Dimitris Schizas
Numan Doctor
MBBS MRCGP MSC BSSM
Dr Michael Lacey
Numan Doctor
MBChB BSc (Hons) MRCGP
Dr Alexandra Davidson
Numan Doctor
BSc (Hons) MBBS MRCGP AFHEA MA (Hons) AFMCP
Abby Watts
Senior Nurse Prescriber
BSc Adult Nursing, IP
