Many men wonder whether testosterone replacement therapy, such as Testogel, can increase penis size in adulthood. This is a common misconception that requires clear, evidence-based clarification. Testogel is a prescription-only transdermal testosterone gel licensed in the UK for treating confirmed hypogonadism—a condition where the testes produce insufficient testosterone. Whilst testosterone plays a critical role in penile development during foetal life and puberty, the biological mechanisms that permit growth are no longer active once puberty is complete. This article explains how Testogel works, the role of testosterone in sexual development, and what realistic outcomes men can expect from properly supervised testosterone replacement therapy.

Understanding Testogel and Testosterone Replacement Therapy

Testogel is a transdermal testosterone gel licensed in the UK for testosterone replacement therapy (TRT) in adult men with confirmed hypogonadism—a condition where the testes produce insufficient testosterone. The gel is applied once daily to clean, dry skin on the shoulders and upper arms only (for Testogel 16.2 mg/g; do not apply to the abdomen or genitals) and delivers testosterone through the skin into the bloodstream, helping to restore physiological testosterone levels.

The Medicines and Healthcare products Regulatory Agency (MHRA) has approved Testogel for specific indications, primarily to address symptoms of testosterone deficiency such as reduced libido, fatigue, decreased muscle mass, mood disturbances, and impaired sexual function. Before initiating treatment, clinicians must confirm low testosterone levels through at least two early-morning blood tests, as testosterone levels naturally fluctuate throughout the day. The National Institute for Health and Care Excellence (NICE) Clinical Knowledge Summary (CKS) and the British Society for Sexual Medicine (BSSM) emphasise that TRT should be reserved for men with confirmed biochemical and symptomatic hypogonadism, not for general 'anti-ageing' purposes or physical enhancement in men with normal testosterone levels.

Mechanism of action: Once absorbed, testosterone binds to androgen receptors throughout the body, influencing protein synthesis, bone density, red blood cell production, and sexual function. In men with genuine hypogonadism, TRT aims to alleviate deficiency symptoms rather than enhance physical characteristics beyond normal physiological parameters.

Important safety information for transdermal gels:

• Application: Apply to shoulders and upper arms only; never to the genitals. Allow the gel to dry fully before dressing.

• Preventing transfer: Wash hands thoroughly with soap and water immediately after application. Cover the application site with clothing once dry. Avoid skin-to-skin contact with others (especially women and children) until the site has been washed. Wait at least 6 hours before showering, bathing, or swimming.

• Flammability: Testogel is alcohol-based and flammable until dry. Avoid fire, flames, or smoking until the gel has dried completely.

• Monitoring: Treatment requires ongoing medical supervision, including baseline and regular blood tests (typically at 3, 6, and 12 months, then annually) to monitor testosterone levels (pre-application 'trough' samples), full blood count (haematocrit/haemoglobin), prostate-specific antigen (PSA), liver function, lipids, and blood pressure. If haematocrit exceeds 54%, treatment should be stopped or adjusted.

Testogel is a prescription-only medicine. Treatment should only commence following thorough clinical assessment, including evaluation of symptoms, blood tests, and exclusion of contraindications such as prostate or breast cancer. Caution is required in men with significant cardiovascular disease, severe heart failure (risk of fluid retention and oedema), or other serious conditions. Evidence on cardiovascular risk with TRT remains inconclusive, and the MHRA advises careful consideration in men with pre-existing heart disease.

Drug interactions: Testosterone may potentiate the effects of anticoagulants (e.g., warfarin—monitor INR closely), affect blood glucose control in men with diabetes (monitor glucose and adjust antidiabetic therapy as needed), and increase fluid retention risk when used with corticosteroids.

Fertility: TRT suppresses natural testosterone production and spermatogenesis, which can impair fertility. Men wishing to conceive should discuss alternative treatments (e.g., human chorionic gonadotrophin or clomiphene) with a specialist before starting TRT.

For full prescribing information, consult the electronic Medicines Compendium (eMC) Summary of Product Characteristics (SmPC) for Testogel 16.2 mg/g and refer to NICE CKS, BSSM guidelines, the British National Formulary (BNF), and NHS resources on testosterone deficiency.

How Testosterone Affects Penis Development and Growth

Testosterone plays a critical role in male sexual development, but this influence is predominantly confined to specific developmental windows. During foetal development and puberty, testosterone and its more potent metabolite, dihydrotestosterone (DHT), are essential for the differentiation and growth of male external genitalia, including the penis.

Foetal development: Between approximately 8 and 20 weeks of gestation, androgens direct the formation of the penis from undifferentiated genital tissue. Inadequate androgen exposure during this critical period can result in micropenis (defined as a stretched penile length more than 2.5 standard deviations below the mean for age) or ambiguous genitalia. Any concerns about genital development in infancy or childhood warrant referral to paediatric endocrinology.

Puberty: The second major growth phase occurs during adolescence, typically between ages 10 and 16, when rising testosterone levels trigger penile growth alongside other secondary sexual characteristics such as increased testicular volume, pubic hair development, voice deepening, and muscle mass increase. During this period, the penis typically grows in both length and girth, reaching adult dimensions by late adolescence or early adulthood.

Post-pubertal period: Once puberty is complete and skeletal maturity is reached (usually by age 18–21), the penis has reached its adult size. At this stage, the developmental window for androgen-driven penile growth has closed. Adult penile tissue does not retain the capacity for further growth in response to testosterone, whether at physiological or supraphysiological levels.

Important exception: In men who experienced delayed or absent puberty due to untreated hypogonadism (e.g., hypogonadotrophic hypogonadism), initiating TRT in late adolescence or early adulthood may lead to some catch-up development of the penis and testes. However, this applies only to individuals who missed normal pubertal androgen exposure, not to men who completed normal puberty.

Medical literature, including guidance from the Endocrine Society and BSSM, consistently demonstrates that testosterone administration after normal puberty does not induce further penile growth. The developmental mechanisms that permitted growth during foetal life and adolescence are no longer active in adulthood, meaning adult penile dimensions remain largely fixed regardless of subsequent testosterone levels.

Will Testogel Cause Penis Growth in Adults?

The straightforward answer is no—Testogel will not cause penis growth in adults who have completed normal puberty. This is one of the most common misconceptions about testosterone replacement therapy, and it is important to address it with clear, evidence-based information.

Once skeletal maturity is reached and puberty is complete, the biological mechanisms that permitted penile growth during development are no longer active. There is no evidence from clinical studies or systematic reviews that TRT increases penile size in adults after normal puberty, regardless of the dose or duration of treatment.

What Testogel can improve in adults with confirmed hypogonadism:

• Erectile function: Restoring normal testosterone levels may improve erectile quality in men with testosterone deficiency, though erectile dysfunction often has multiple causes (vascular, neurological, psychological) that may require additional assessment and treatment.

• Sexual desire (libido): Low testosterone is associated with reduced libido, which often improves with appropriate TRT.

• Overall well-being: Addressing genuine hormone deficiency can enhance mood, energy, and psychological well-being, which may indirectly benefit sexual relationships and confidence.

However, these improvements relate to sexual function and hormonal balance, not anatomical growth. It is crucial to have realistic expectations before starting TRT. Men seeking Testogel specifically for penile enlargement will be disappointed, as this is not a recognised or achievable outcome of the treatment.

Important safety considerations:

Using Testogel without medical supervision or in the absence of confirmed hypogonadism carries significant risks. Common and serious adverse effects include:

• Application-site reactions (redness, irritation, rash)

• Acne and oily skin

• Polycythaemia (excessive red blood cell production), which increases the risk of blood clots

• Fluid retention and oedema, particularly in men with heart, kidney, or liver disease

• Mood disturbances (irritability, aggression, mood swings)

• Suppression of natural testosterone production and impaired fertility

• Risk of secondary exposure to women and children through skin contact, which can cause virilisation in women and premature sexual development in children

Evidence on cardiovascular risk with TRT is mixed and inconclusive. The MHRA advises caution in men with significant cardiovascular disease, and careful monitoring is essential.

The MHRA and NHS strongly advise against obtaining testosterone products without proper medical assessment and prescription. If you experience any suspected side effects, report them via the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or through the Yellow Card app.

Realistic Expectations and Medical Guidance for Testogel Treatment

If you are considering Testogel, it is essential to approach treatment with evidence-based expectations and proper medical guidance. Testosterone replacement therapy offers genuine benefits for men with confirmed hypogonadism, but it is not a solution for anatomical concerns or a performance enhancer for men with normal testosterone levels.

Appropriate reasons to consider TRT:

• Confirmed low testosterone levels on two separate early-morning blood tests (typically below 8–12 nmol/L, though reference ranges vary; NICE CKS recommends repeat testing to confirm biochemical deficiency)

• Symptoms consistent with hypogonadism: persistent fatigue, reduced libido, erectile dysfunction, decreased muscle mass, mood changes, or reduced bone density

• Exclusion of other causes for symptoms (thyroid disorders, depression, sleep apnoea, diabetes, hyperprolactinaemia)

What to expect from properly supervised TRT:

Men with genuine testosterone deficiency often experience improvements in energy levels, mood, sexual function, muscle mass, and bone density over several months of treatment. However, response varies individually, and not all symptoms resolve completely.

Monitoring and follow-up:

Regular monitoring is essential to ensure safety and efficacy. Your doctor will arrange blood tests:

• Baseline: Testosterone (two early-morning samples), full blood count, PSA (and digital rectal examination if appropriate), liver function, lipids, blood pressure, and blood glucose.

• Follow-up: Testosterone levels should be checked 14–28 days after starting or adjusting the dose (pre-application 'trough' sample, taken before applying the gel). Aim for testosterone levels in the mid-normal range. Full blood count, PSA, liver function, lipids, and blood pressure should be monitored at 3, 6, and 12 months, then annually.

• Haematocrit monitoring: If haematocrit rises above 54%, treatment should be stopped or the dose reduced, as this increases the risk of blood clots.

When to seek medical advice and red-flag symptoms:

If you have concerns about sexual function, penis size, or symptoms potentially related to low testosterone, consult your GP rather than self-diagnosing or seeking treatment independently. Your doctor can arrange appropriate investigations and, if indicated, refer you to an endocrinologist or urologist.

Red flags requiring urgent specialist referral:

• Very low testosterone with low or normal luteinising hormone (LH) and follicle-stimulating hormone (FSH), which may indicate pituitary disease

• Symptoms of hyperprolactinaemia (e.g., breast discharge, headaches, visual disturbances)—refer urgently to endocrinology

• Sudden penile curvature, pain, or deformity (possible Peyronie's disease)—refer to urology

• Suspected prostate cancer (abnormal PSA or digital rectal examination)—refer urgently to urology

For concerns specifically about penis size, it is worth noting that perceived size concerns are often not reflective of actual measurements, and psychological support may be more appropriate than medical intervention. According to a systematic review published in the British Journal of Urology International (BJUI), the average erect penis length in studies is approximately 13–14 cm (measured from the pubic bone to the tip), with considerable normal variation. If you have concerns, discuss them with your GP, who can provide reassurance or refer you to urology if clinically indicated.

Fertility considerations:

TRT suppresses the body's natural production of testosterone and sperm. If you are planning to have children, discuss this with your doctor before starting treatment. Alternative treatments (e.g., human chorionic gonadotrophin or clomiphene citrate) may be more appropriate and should be managed under specialist care.

Final guidance:

Testogel is an effective, evidence-based treatment for testosterone deficiency when used appropriately under medical supervision. However, it will not cause penis growth in adults who have completed normal puberty. Setting realistic expectations and working closely with healthcare professionals ensures safe, beneficial treatment outcomes whilst avoiding disappointment or potential harm from inappropriate use.

For further information, consult the eMC SmPC for Testogel, NICE CKS on testosterone deficiency, BSSM guidelines, the BNF, NHS pages on hypogonadism and erectile dysfunction, and the European Medicines Agency (EMA) European Public Assessment Report (EPAR) for Testogel.

Frequently Asked Questions