Testosterone replacement therapy (TRT) can produce real benefits for men with confirmed low testosterone, but it’s not a risk-free treatment, and it’s not appropriate for every man. Understanding the actual side effects — both the common, manageable ones and the more serious risks that require ongoing monitoring — is just as important as understanding the benefits before starting treatment.
This guide covers what the evidence shows about TRT’s risks and how doctors monitor for them. For the other side of this picture, see our guide on the benefits of testosterone therapy, and for an explanation of how treatment itself works, see how does TRT work.
This article is for general information and isn’t a substitute for a medical evaluation. TRT should only be started and monitored under the supervision of a doctor.
Common, Milder Side Effects
These side effects are relatively frequent and generally manageable, though they’re worth discussing with a doctor if they occur.
Skin reactions
- Injection site reactions: redness, soreness, or minor swelling at the injection site, generally temporary
- Skin irritation from patches or gels: contact dermatitis at the application site is a common reason men switch delivery methods
- Acne and oily skin: testosterone increases oil gland activity, and some men notice more breakouts, particularly on the back and shoulders
Fluid retention
Mild fluid retention, sometimes causing slight swelling in the ankles or hands, can occur, particularly at higher doses.
Mood fluctuations
Especially with injections, some men notice mood or energy fluctuations that track with the rise and fall of testosterone levels between doses — often described as feeling noticeably better right after an injection and somewhat less so as the next dose approaches. This is one reason some men prefer more frequent, smaller doses or a steadier delivery method like gel.
Elevated Red Blood Cell Count (Erythrocytosis)
This is one of the more clinically significant risks of TRT and a primary reason for routine blood monitoring. Testosterone stimulates red blood cell production, and in some men, levels rise more than intended, thickening the blood and increasing the theoretical risk of blood clots, stroke, or cardiovascular events. This is why hematocrit (the percentage of blood volume made up of red blood cells) is checked regularly during treatment, and why a dose adjustment, temporary pause, or occasionally a therapeutic blood donation may be recommended if levels climb too high.
Prostate Considerations
Testosterone’s relationship with the prostate has been debated for decades. Current evidence does not show that TRT causes prostate cancer, but it can accelerate the growth of an existing, undetected prostate cancer, which is why prostate health is screened before starting treatment and monitored periodically afterward, particularly in men over 40 or with other risk factors. This typically includes a PSA (prostate-specific antigen) blood test and, depending on age and risk factors, a physical exam.
Fertility and Testicular Changes
Because external testosterone signals the brain to reduce its own hormone production, TRT typically suppresses natural testosterone and sperm production over time. Practical effects include:
- Reduced sperm count, sometimes significant enough to cause temporary infertility while on treatment
- Testicular shrinkage, since reduced stimulation from the brain’s signaling hormones causes the testes to produce less and, over time, become smaller
These effects are generally reversible after stopping TRT, though recovery time varies and isn’t guaranteed to return fully in every case, particularly with long-term use. Men who want to preserve fertility while addressing low testosterone often consider Clomid instead, which stimulates the body’s own production rather than replacing it — see our guide on what is Clomid therapy for that comparison.
Gynecomastia and Estrogen-Related Effects
Some of the testosterone introduced through TRT converts into estrogen through a natural enzyme process. In most men, this stays balanced, but in some, particularly those with higher body fat or a genetic tendency toward higher conversion, estrogen levels can rise enough to cause:
- Breast tissue enlargement or tenderness (gynecomastia)
- Water retention
- Mood changes
Doctors sometimes monitor estradiol (the primary estrogen marker) alongside testosterone, particularly if these symptoms develop.
Sleep Apnea
TRT can worsen sleep apnea in men who already have it, or in some cases unmask a previously undiagnosed case. This is one reason doctors typically ask about sleep symptoms, including snoring, daytime fatigue, and witnessed breathing pauses, before starting treatment, and may recommend a sleep evaluation if these symptoms are present.
Cardiovascular Considerations
Research on TRT and cardiovascular risk has produced mixed results over the years, with some earlier studies raising concern about increased cardiovascular events and more recent, larger trials generally showing no significant increase in risk for appropriately selected, monitored patients. This remains an area of ongoing research, and it’s a key reason TRT is generally reserved for men with confirmed low testosterone and monitored closely, rather than used more broadly. Men with a history of heart attack, stroke, or uncontrolled cardiovascular disease should discuss this risk profile carefully with a doctor before starting.
Who Should Be Cautious About or Avoid TRT
TRT is generally not recommended, or requires extra caution, for men with:
- Active or suspected prostate cancer
- Untreated severe sleep apnea
- A history of blood clots or significantly elevated hematocrit
- Uncontrolled heart failure
- Desire to preserve fertility in the near term
- Breast cancer (a rare but documented consideration in men)
A full medical history and physical exam before starting treatment is designed to catch these contraindications ahead of time.
What Monitoring Actually Looks Like
Routine monitoring during TRT typically includes:
- Testosterone levels, to confirm the dose is producing a healthy, stable range
- Hematocrit or complete blood count, to monitor for elevated red blood cell production
- PSA, to monitor prostate health, particularly in men over 40
- Estradiol, in men who develop symptoms suggesting elevated estrogen
- Lipid panel, since testosterone can affect cholesterol levels in some men
- Symptom review, covering mood, sleep, libido, and any new physical changes
This monitoring typically happens more frequently in the first several months of treatment, then settles into a periodic schedule (often every six to twelve months) once a stable, well-tolerated dose is established.
Weighing Risks Against Benefits
None of this is meant to suggest TRT is inherently dangerous for the right candidate — for men with confirmed low testosterone and appropriate monitoring, the treatment has a long track record of use with a generally manageable side effect profile. The goal of understanding these risks is to make an informed decision, not to avoid treatment altogether. Reading this alongside our guide on the benefits of testosterone therapy gives a fuller, more balanced picture before starting.
When to Contact a Doctor
Reach out promptly if you notice, while on TRT:
- Swelling in the legs, chest pain, or shortness of breath
- New or worsening breast tenderness or enlargement
- Significant mood changes or increased irritability
- Signs of worsening sleep apnea, such as loud snoring or excessive daytime fatigue
- Any symptom that feels unusual or concerning since starting treatment
Routine follow-up appointments are also the time to raise smaller concerns, like acne or injection site irritation, so adjustments can be made before they become bigger issues.
Where This Leaves You
TRT carries real, well-documented risks — from elevated red blood cell counts and prostate considerations to fertility effects and, in some men, mood or skin changes — but most are manageable with proper screening and ongoing monitoring. Understanding these risks in full, rather than only the benefits, is what allows a doctor and patient to make a genuinely informed decision together about whether TRT is the right treatment.
