Testosterone replacement therapy (TRT) is an effective, well-established treatment for men with a confirmed diagnosis of low testosterone, but like any prescription medicine it carries side effects. Some are common and easily managed, some need closer monitoring, and a small number rule TRT out for certain men altogether. This guide sets out what to expect, what clinicians actually check for, and how side effects are managed in UK practice, rather than leaving you to piece it together from forum threads.
None of this is a reason to be alarmed about TRT as a treatment. It's a reason monitoring exists in the first place: the side effects below are, for the most part, predictable, well understood and manageable precisely because they're looked for on a schedule rather than left to chance.
The common side effects
Most men who start TRT notice side effects that are mild and predictable rather than alarming. The most frequently reported include:
- Acne or oilier skin
- Fluid retention, which can show up as mild ankle or facial puffiness
- Injection-site reactions, such as soreness or a small lump, for men on injectable testosterone
- Breast tenderness, which happens because some testosterone naturally converts into oestradiol in the body
None of these are signs that treatment is going wrong. They're recognised, expected effects that a clinician will ask about at every review, and they can usually be reduced with a change in dose, formulation or injection technique.
Raised haematocrit: why it matters
One of the most closely watched effects of TRT is a rise in haematocrit, the proportion of your blood made up of red blood cells. Testosterone stimulates red cell production, and in some men this pushes haematocrit high enough to thicken the blood meaningfully. This is checked by a routine full blood count at every monitoring appointment, and a level above 54% is generally the threshold that prompts action: usually a change in dose, a longer interval between injections, or occasionally a referral for venesection, where blood is drawn to bring the level back down. Catching this early is the entire point of monitoring. A raised haematocrit that's picked up and corrected is a manageable finding, not a reason to stop treatment outright.
Fertility and TRT
TRT affects the body's own hormone signalling in a way that commonly suppresses sperm production. Testosterone from an outside source tells the pituitary gland that testosterone levels are already high, which lowers its output of LH and FSH, the two hormones that drive the testes to produce both testosterone and sperm. The practical effect is that sperm counts often fall, sometimes to very low levels, while you're on TRT. This is usually reversible once treatment stops, though recovery takes time and isn't always complete for every man, depending on how long he's been on treatment and his starting fertility. If starting a family is part of your plans now or in the next few years, this is worth raising before you start treatment, not after. Our guide to TRT and fertility covers the options, including fertility preservation, in more detail. Some men choose to bank sperm before starting, while others monitor fertility markers alongside their usual TRT bloods and revisit the decision if their plans change.
Mood, sleep apnoea and fluid retention
Mood effects on TRT are genuinely individual. Many men with confirmed low testosterone report improved mood and energy once treated, but testosterone can also worsen irritability or anxiety in some men, particularly if the dose runs high between blood tests. Fluid retention, mentioned above as a common early effect, is usually mild on its own. A more significant point is that TRT can worsen untreated obstructive sleep apnoea, a condition where breathing repeatedly stops and starts during sleep. If you snore heavily, wake up gasping, or have ever been told you might have sleep apnoea, it's worth mentioning before starting TRT, since an avoidable worsening of that condition is easy to prevent with a straightforward conversation up front.
Prostate and heart: what the evidence says
Two of the longest-standing worries about testosterone, prostate cancer and heart disease, deserve a precise answer rather than a vague reassurance. TRT is contraindicated in men with active prostate or breast cancer, and PSA, a prostate blood marker, is monitored throughout treatment as standard practice, particularly in older men, as a precaution. Current evidence doesn't show that TRT causes prostate cancer in men without pre-existing disease; monitoring exists because the prostate is testosterone-sensitive tissue, not because of a proven causal link. On the heart, the TRAVERSE trial, a large 2023 randomised study, found no increase in major cardiovascular events with testosterone compared with placebo in men with hypogonadism, among the most reassuring pieces of direct evidence available on this question so far. That doesn't mean cardiovascular risk factors are ignored: blood pressure, lipids and any existing cardiovascular disease are still considered individually as part of ongoing care.
How side effects are monitored and managed
What turns TRT's side effects from a list of risks into a managed treatment is a structured monitoring schedule, not luck. UK clinicians typically review testosterone levels, haematocrit and PSA, where age-appropriate, around 3 months after starting or after any dose change, again at 6 months, then every 6 to 12 months once things are stable, alongside a conversation about any symptoms you've noticed. Our guide to TRT blood tests explains what's actually checked and why.
If a problem does show up, the response is usually straightforward: adjusting the dose, changing the interval between injections or switching formulation, or, for raised haematocrit specifically, arranging venesection. Our guide to how TRT dose is set and adjusted covers this process step by step. None of this works if monitoring itself is skipped or treated as optional, which is exactly why it isn't optional in a properly run TRT service, whatever the price difference between providers might suggest. A programme that doesn't repeat bloods on a defined schedule has removed the exact mechanism that keeps TRT's side effects manageable in the first place.
It's also worth remembering that most side effects, even the ones that sound concerning on paper, are picked up well before they cause lasting harm, precisely because they're looked for routinely rather than waited for. If something changes between your scheduled appointments, such as new breathlessness, unusual bruising, or a symptom that worries you, it's always reasonable to contact your clinician sooner rather than waiting for the next review.
If you're currently on TRT and weighing up whether it still makes sense for you, or you're considering starting it and want a clearer picture of what you're taking on, our broader guide to whether TRT is safe is worth reading alongside this one. A UK-registered clinician can also go through your own situation and bloods directly, and help you weigh these side effects against the symptoms that brought you to TRT in the first place.