TestoGel is a prescription testosterone replacement therapy licensed in the UK exclusively for treating male hypogonadism—a medical condition characterised by abnormally low testosterone levels. Whilst clinical evidence demonstrates that TestoGel can increase lean body mass in men with confirmed testosterone deficiency, typically by 1.6–2.5 kg over 12 months, it is not licensed or indicated for muscle building in men with normal testosterone levels. The extent of muscle mass changes depends on multiple factors, including baseline testosterone levels, age, adherence to treatment, and crucially, engagement in resistance training. This article examines the clinical evidence, influencing factors, and NHS guidance on safe use of TestoGel for hypogonadism.

What Is TestoGel and How Does It Work?

TestoGel is a transdermal testosterone replacement therapy (TRT) licensed in the UK for treating male hypogonadism—a condition characterised by abnormally low testosterone levels. Available as a clear, colourless gel containing testosterone, it comes in different formulations (1% or 16.2 mg/g). The 1% formulation is applied daily to clean, dry skin on the shoulders, upper arms, or abdomen, while the 16.2 mg/g formulation should only be applied to shoulders and upper arms. The active ingredient is absorbed through the skin into the bloodstream, providing a steady release of testosterone throughout the day.

The mechanism of action centres on restoring physiological testosterone levels rather than elevating them beyond normal ranges. Once absorbed, testosterone binds to androgen receptors in various tissues, including skeletal muscle, where it influences protein synthesis and nitrogen retention. Testosterone also reduces muscle protein breakdown—processes fundamental to muscle maintenance and growth.

It is crucial to understand that TestoGel is not licensed as a performance-enhancing drug or bodybuilding supplement. The Medicines and Healthcare products Regulatory Agency (MHRA) has approved it strictly for men with confirmed hypogonadism, diagnosed through repeated blood tests showing low testosterone levels alongside relevant symptoms such as reduced libido, fatigue, and loss of muscle mass. Prescribing TestoGel to men with normal testosterone levels is considered off-label use and carries potential health risks.

The gel formulation offers advantages over some testosterone delivery methods, including avoidance of injections and more stable blood testosterone levels compared to short-acting intramuscular preparations. However, users must take precautions to prevent transfer to others, particularly women and children, by covering application sites with clothing and washing hands thoroughly after application. The gel is flammable until dry, should not be applied to the genitals, and users should avoid washing the application site or swimming for the period specified in the product information (typically 2-6 hours depending on formulation).

Clinical Evidence on Testosterone Gel and Muscle Growth

Clinical research demonstrates that testosterone replacement therapy, including gel formulations, can increase lean body mass in men with hypogonadism, though the magnitude of effect varies considerably. A systematic review published in the Journal of Clinical Endocrinology & Metabolism found that TRT typically increases lean mass by approximately 1.6–2.5 kg over 12 months in hypogonadal men, with corresponding reductions in fat mass.

However, it is essential to distinguish between lean body mass and functional muscle strength. Whilst TestoGel reliably increases lean tissue, studies show more modest improvements in muscle strength and physical performance. Research indicates that strength gains are often smaller than the increase in muscle size would predict, suggesting that testosterone's effects on muscle quality and neuromuscular function may be limited when used at replacement doses.

Key findings from clinical trials include:

• The Testosterone Trials (TTrials), a large US study, showed modest improvements in sexual function and mood but limited effects on physical function in older men with low testosterone

• European studies confirm that younger hypogonadal men may experience more pronounced muscle mass gains than older populations

• Muscle benefits appear greatest in the first 6–12 months of treatment, with effects often plateauing thereafter

Importantly, there is no official evidence supporting the use of testosterone gel for muscle building in men with normal testosterone levels. The European Medicines Agency (EMA) and NICE emphasise that TRT should only be prescribed for confirmed hypogonadism. Using TestoGel without medical indication does not produce the dramatic muscle gains some might expect and carries health risks, including possible blood pressure increases, polycythaemia (increased red blood cells), oedema, and suppression of natural testosterone production and fertility. The EMA has concluded there is no consistent evidence of increased cardiovascular risk when testosterone is used as indicated for hypogonadism.

Factors That Influence Muscle Mass Changes with TestoGel

The extent to which TestoGel increases muscle mass depends on multiple interacting factors, making individual responses highly variable. Understanding these influences helps set realistic expectations and optimise treatment outcomes.

Baseline testosterone levels represent the most significant predictor. Men with severely deficient testosterone (typically <8 nmol/L) tend to experience more noticeable improvements in body composition than those with borderline low levels (8-12 nmol/L). The body's response follows a dose-response relationship up to physiological levels, but exceeding normal ranges does not produce proportionally greater benefits and increases adverse effect risks.

Age and treatment duration also play crucial roles. Responses may be attenuated in older men compared to younger individuals with hypogonadism, possibly due to age-related changes in androgen receptor sensitivity and presence of comorbidities. Most muscle accrual occurs within the first year of treatment, with diminishing returns thereafter.

Lifestyle factors significantly modulate testosterone's anabolic effects:

• Resistance training: Testosterone's muscle-building effects are substantially enhanced by regular strength training. Studies show that TRT combined with structured exercise produces significantly greater muscle gains than TRT alone

• Nutrition: Adequate protein intake supports muscle protein synthesis

• Sleep quality: Poor sleep impairs muscle recovery and may blunt testosterone's anabolic effects

• Body composition: Men with higher baseline body fat may experience attenuated responses, as adipose tissue can convert testosterone to oestrogen

Management of comorbidities such as obesity, obstructive sleep apnoea, and diabetes can optimise treatment outcomes. Adherence to treatment is critical. Inconsistent application or premature discontinuation prevents achievement of stable testosterone levels necessary for sustained muscle benefits. Additionally, individual responses may vary due to differences in testosterone metabolism and tissue sensitivity.

Safe Use of TestoGel: NHS Guidelines and Monitoring

The NHS and NICE provide clear guidance on the safe prescribing and monitoring of testosterone replacement therapy to maximise benefits whilst minimising risks. TestoGel should only be initiated by specialists or GPs with appropriate expertise in managing hypogonadism.

Before starting treatment, patients require:

• Two separate blood tests (taken before 10 am) confirming low testosterone levels, typically at least one week apart

• Assessment of symptoms consistent with testosterone deficiency

• Measurement of LH, FSH, and prolactin (and SHBG where indicated) to help determine the cause

• Prostate examination and PSA (prostate-specific antigen) testing in men over 50 or those at increased risk of prostate disease

• Full blood count to check haematocrit levels

• Blood pressure measurement

• Exclusion of contraindications, including prostate or breast cancer

Ongoing monitoring is essential for safe long-term use. NICE recommends:

• Testosterone levels checked at 3 months, then annually once stable, with dose adjustments to maintain levels in the mid-normal range according to local laboratory reference ranges

• Full blood count at 3 months, then annually, to detect polycythaemia (haematocrit >0.54 requires dose reduction or treatment interruption)

• PSA and prostate assessment annually in men over 50 or those at increased risk

• Blood pressure monitoring periodically

Patient safety advice includes reporting symptoms such as:

• Ankle swelling or breathing difficulties (potential heart failure)

• Prolonged or frequent erections

• Mood changes, aggression, or sleep disturbances

• Urinary symptoms suggesting prostate enlargement

Patients should contact their GP promptly if these occur and report suspected side effects via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk). It is also vital to prevent gel transfer by applying to areas covered by clothing, allowing drying time before dressing, and avoiding skin-to-skin contact with others for several hours post-application. The application site should be washed before anticipated close contact. Women (especially pregnant women) and children accidentally exposed should wash the contact area immediately with soap and water.

Patients should be advised that TestoGel suppresses sperm production and is not suitable for men trying to conceive. TestoGel represents an effective treatment for confirmed hypogonadism when used appropriately under medical supervision, but it is not a solution for muscle building in healthy men.

Frequently Asked Questions