TRT Side Effects and Monitoring: What to Expect
The side effects of testosterone replacement therapy, the blood tests that keep it safe, and the warning signs to act on.
Part of the Access Doctor Low Testosterone Guide.
Key fact: Most men tolerate TRT well, but it can thicken the blood and stimulate the prostate. That's why testosterone, haematocrit and PSA are checked at about 3 to 6 months, at 12 months, and then every year.
Testosterone replacement therapy (TRT) is usually long-term. Knowing which side effects to expect, and which blood tests are due when, helps you stay on treatment safely and spot problems early. For the condition itself, see our guide to low testosterone.
Due a Blood Test?
Check your testosterone level at home between reviews with a morning finger-prick test analysed by a UK lab.
Order a Testosterone Blood Test →Common Side Effects
- Skin reactions where you apply the gel: redness, itching, dryness or acne
- Acne and oily skin elsewhere
- Breast tenderness or enlargement
- Ankle swelling from fluid retention
- Raised blood pressure
- Mood changes, such as irritability
What helps
| Side effect | What to do |
|---|---|
| Skin reactions | Change the exact spot each day, within the area your gel is licensed for. If it carries on, ask about a different gel |
| Acne | Wash gently twice a day and avoid oily skin products. A pharmacist can suggest treatment; see your GP if it’s severe or scarring |
| Breast tenderness or enlargement | Tell your prescriber. It may settle if your dose is lowered. Any lump or nipple discharge needs a GP check |
| Mood changes | Tell your prescriber: your level may be too high. Get help straight away if your mood is very low |
| Ankle swelling | Tell your prescriber. See a doctor urgently if you’re also breathless |
Haematocrit: Thicker Blood
Testosterone stimulates red blood cell production. If the haematocrit (the proportion of the blood made up of red cells) rises too high, the blood gets thicker and the risk of clots goes up.
In plain terms, a haematocrit of 0.54 means 54% of your blood is red cells. What happens depends on the result:
- Below 0.52: carry on, with your usual checks
- 0.52 to 0.54: a review. Your dose may be lowered and the test repeated, and causes such as smoking, dehydration and sleep apnoea are looked for
- Above 0.54: treatment is paused until it falls, then restarted at a lower dose if appropriate. Occasionally a specialist arranges for some blood to be removed (venesection)
Prostate and PSA
Testosterone can stimulate the prostate, so PSA is checked before treatment and regularly afterwards, usually in men aged 40 and over. Large studies haven’t shown that TRT causes prostate cancer, but it isn’t used in men who have it. A PSA of 3 or more, or a rise of more than 1.4 in a year, usually means treatment is stopped and you’re referred for urology review. New urinary symptoms should also be checked.
Home test
PSA Blood Test
A prostate (PSA) test for men who've chosen to have one, or are on TRT.
View product →Home test
Testosterone Blood Test
A morning finger-prick test, analysed by a UK lab.
View product →Heart, Blood Pressure and Blood Clots
A large 2023 trial (TRAVERSE) in men with low testosterone and heart risk factors found that testosterone gel didn’t increase heart attacks or strokes compared with placebo. Atrial fibrillation and blood clots in the lungs were slightly more common, so tell your prescriber about any heart rhythm problem or past clot. Blood pressure should be checked at least once a year.
This followed earlier concern about heart risk. A European Medicines Agency review in 2014 found no consistent evidence that testosterone increases heart problems in men with confirmed low testosterone. If you have heart disease, your GP or specialist should be involved in decisions about TRT, and it isn’t used within 6 months of a heart attack or stroke or in uncontrolled heart failure.
Sleep Apnoea
Sleep apnoea, where breathing repeatedly pauses during sleep, and low testosterone often go together: both are linked with being overweight, and poor sleep itself lowers testosterone. Testosterone treatment can make untreated sleep apnoea worse, and sleep apnoea raises haematocrit.
- Signs to look for: loud snoring, pauses in breathing or gasping noticed by a partner, waking unrefreshed, morning headaches and falling asleep in the day
- What to do: see your GP, who can arrange a sleep study
- If it’s treated (for example with CPAP), TRT can usually continue with monitoring
Fertility
TRT lowers sperm production and can make it harder to conceive. It isn’t suitable if you’re trying for a baby. If you want children, talk to your GP or specialist before starting or continuing, as other treatments can be used.
Testosterone treatment switches off the signals from the brain (LH and FSH) that tell the testicles to make sperm. Sperm counts can fall very low within a few months. They usually recover after stopping, often within 6 to 12 months, but it can take longer and isn’t guaranteed.
- TRT isn’t contraception: some men stay fertile on it
- Planning a family later? Ask about freezing sperm before you start
- Want children now? A specialist can use other medicines that raise your own testosterone while protecting sperm production
Other Medicines
- Blood thinners such as warfarin: testosterone can increase their effect, so you may need extra blood tests when starting or changing dose
- Insulin and other diabetes medicines: testosterone can lower blood sugar, so doses may need adjusting
- Steroid tablets: taken together, fluid retention is more likely
Tell your prescriber about everything you take, including supplements and anything bought for bodybuilding.
Your Monitoring Schedule
| When | What’s checked |
|---|---|
| Before starting | Testosterone (twice), full blood count, PSA, liver tests, cholesterol, blood sugar, blood pressure |
| 2 to 8 weeks | Testosterone level, to set the dose (timing depends on the gel) |
| 3 to 6 months | Testosterone, haematocrit, PSA and how you feel |
| 12 months | Testosterone, haematocrit, PSA, liver tests, blood pressure and weight |
| Every year after | The same checks as at 12 months |
When to take the testosterone test depends on your gel:
| Your gel | When to take the blood test |
|---|---|
| Testogel (pump or sachets) | In the morning, before you apply that day’s dose |
| Testavan | 2 to 4 hours after applying it |
| Tostran | About 2 hours after applying it |
To order a repeat prescription from us, you’ll need results from the last 12 months. See testosterone blood test for timing.
Stopping TRT
While you’re on TRT, your body makes less of its own testosterone. If you stop, your level falls over a few weeks and symptoms such as tiredness, low mood and low sex drive may return. Your own production may take weeks or months to recover, and if the original cause is still there, your level will settle back to where it was before treatment.
Stopping isn’t dangerous, but talk to your prescriber first, especially if you’re stopping because of side effects, a blood result or plans for a family. They can advise on timing and follow-up blood tests.
When to Get Help
Call 999 or go to A&E if you have chest pain, sudden breathlessness, a painful swollen leg, signs of a stroke (face drooping, arm weakness, speech problems), or a painful erection lasting more than 4 hours.
See your GP if you have new urinary symptoms, breast changes, ankle swelling, severe headaches, or low mood. Report suspected side effects through the MHRA Yellow Card scheme.
Already on Testosterone Treatment?
Order your repeat prescription online. A UK pharmacist prescriber checks your treatment and recent blood results. For men already prescribed testosterone only.
Start Low Testosterone Consultation →Frequently Asked Questions
What are the most common side effects of TRT?
Skin reactions where the gel is applied, acne, breast tenderness, ankle swelling and a rise in red blood cells. Most are mild, and blood tests pick up the ones you can't feel.
What haematocrit is too high on TRT?
Above 0.54, treatment is paused until it falls, then restarted at a lower dose if appropriate. Results between 0.52 and 0.54 are reviewed, and the dose may be lowered.
Does TRT cause prostate cancer?
Large studies haven't shown that it does. But testosterone isn't used in men with prostate cancer, so PSA is checked before treatment and regularly during it.
Is TRT bad for your heart?
A large 2023 trial found testosterone gel didn't increase heart attacks or strokes in men with low testosterone. Atrial fibrillation and lung clots were slightly more common, so heart history is checked first.
Can I have children while on TRT?
TRT lowers sperm production and can make conception difficult. If you're planning a family, speak to your GP or specialist about stopping or switching to another treatment.
How often do I need blood tests on TRT?
At about 3 to 6 months and 12 months after starting, then every year: testosterone, haematocrit and PSA, plus blood pressure and weight.
What happens if I stop TRT?
Your testosterone falls over a few weeks and symptoms may return. Your body's own production can take weeks or months to recover. Speak to your prescriber before stopping.
Does TRT make sleep apnoea worse?
It can, if the sleep apnoea is untreated. Loud snoring, pauses in breathing at night and daytime sleepiness should be checked by your GP. Once sleep apnoea is treated, TRT can usually continue.
References
- Hackett G, et al. British Society for Sexual Medicine. British Society for Sexual Medicine guidelines on male adult testosterone deficiency, with statements for practice. World Journal of Men's Health, 2023. bssm.org.uk
- NHS South Yorkshire ICB. Shared care protocol: testosterone for male hypogonadism (Doncaster). December 2025. southyorkshire.icb.nhs.uk
- Electronic Medicines Compendium. Testogel 16.2mg/g gel: Summary of Product Characteristics. medicines.org.uk
- Electronic Medicines Compendium. Testavan 20mg/g transdermal gel: Summary of Product Characteristics. medicines.org.uk
- Electronic Medicines Compendium. Tostran 20mg/g transdermal gel: Summary of Product Characteristics. medicines.org.uk
- MHRA. Yellow Card scheme. yellowcard.mhra.gov.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Prescription medicines require a consultation before they can be dispensed. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.