Many men wonder whether testosterone treatment can increase penis size in adulthood. Whilst testosterone is essential for penile development during puberty, the relationship between this hormone and genital growth changes significantly once sexual maturation is complete. This article examines the medical evidence surrounding testosterone therapy and penile size, explains when hormone treatment may be appropriate, and provides guidance on realistic expectations. Understanding the science behind testosterone's role in development helps clarify common misconceptions and ensures men can make informed decisions about their health.

Understanding Testosterone and Penile Development

Testosterone is the primary male sex hormone (androgen) responsible for the development of male sexual characteristics during puberty. Produced mainly in the testes, testosterone plays a crucial role in penile growth, which occurs predominantly during two key developmental windows: in utero and during puberty.

During foetal development, testosterone is converted to dihydrotestosterone (DHT) by the enzyme 5-alpha-reductase. It is primarily DHT that drives the formation of male external genitalia between weeks 8 and 24 of gestation. The second critical period occurs during puberty, typically between ages 10 and 16, when rising testosterone levels trigger significant penile growth alongside other secondary sexual characteristics such as voice deepening, facial hair development, and increased muscle mass.

The mechanism of testosterone-induced penile growth involves the hormone binding to androgen receptors in penile tissue, stimulating cell proliferation and tissue expansion. This process is most effective when tissues retain their developmental plasticity during puberty. By late adolescence (usually by age 18–21), penile development is generally complete, though there is considerable normal variation in timing.

It is important to understand that normal testosterone levels in adult men typically range from approximately 10 to 30 nmol/L, though reference ranges vary between laboratories and should be interpreted in clinical context. Low testosterone (hypogonadism) in adults can affect sexual function, mood, and energy levels, but the relationship between adult testosterone levels and penile size is fundamentally different from that during developmental periods. Once sexual maturation is complete, the biological mechanisms that govern genital growth are no longer active in the same way.

Can Testosterone Treatment Increase Penis Size in Adults?

The straightforward answer, supported by current medical evidence, is that testosterone treatment does not increase penis size in adults who have completed puberty. Once the developmental windows have closed and sexual maturation is complete, the penile tissues lose their capacity for testosterone-induced growth.

Evidence does not support increases in penile size with testosterone replacement therapy (TRT) in adults who have completed puberty. The cellular mechanisms that allow testosterone to stimulate penile growth during puberty are dependent on developmental factors that are no longer present after maturation. Adult penile tissue, whilst responsive to testosterone for maintaining function, does not retain the growth potential seen in adolescence.

The exception to this rule involves specific medical conditions diagnosed in childhood or adolescence. Boys with hypogonadism (significantly low testosterone production) or delayed puberty may benefit from testosterone treatment during their developmental years. In these cases, appropriately timed hormone therapy before or during puberty can help achieve normal genital development that would otherwise be impaired. However, this intervention must occur before developmental potential is lost.

Some men report subjective improvements in penile appearance during testosterone treatment, but these are typically related to:

• Improved erectile function, making the penis appear fuller

• Reduced suprapubic fat deposits, which can make the penis appear larger

• Increased confidence and body image, affecting perception

• Better overall sexual function, including libido and performance

These changes reflect functional improvements rather than actual tissue growth. It is important to note that erectile dysfunction is usually treated first-line with PDE5 inhibitors (such as sildenafil); TRT is reserved for men with confirmed hypogonadism. It is crucial for patients to have realistic expectations when considering testosterone therapy, understanding that any treatment should be prescribed for clinically appropriate indications rather than cosmetic concerns.

What to Expect from Testosterone Treatment

Testosterone replacement therapy (TRT) is a legitimate medical treatment prescribed for men with clinically diagnosed hypogonadism—a condition where the body produces insufficient testosterone. The MHRA-approved indications for TRT include confirmed testosterone deficiency with associated symptoms such as reduced libido, erectile dysfunction, fatigue, decreased muscle mass, and mood disturbances.

Before initiating treatment, your GP or endocrinologist will require:

• Two separate blood tests (taken in the morning when testosterone levels peak) showing low testosterone levels (typically below 8 nmol/L is considered low, while 8–12 nmol/L may be borderline depending on symptoms)

• Clinical assessment of symptoms

• Additional hormone tests including luteinising hormone (LH), follicle-stimulating hormone (FSH), sex hormone binding globulin (SHBG), and prolactin

• Exclusion of contraindications, including prostate cancer, male breast cancer, or hypersensitivity to the medication

• Baseline investigations, including prostate-specific antigen (PSA), full blood count, and liver function tests

Realistic benefits of TRT in men with genuine hypogonadism include improved sexual function (libido within weeks, erectile quality over months), increased energy levels, better mood and cognitive function, increased muscle mass, and improved bone density (typically after 6–12 months or longer). These effects develop gradually over different timeframes.

Common adverse effects that patients should be aware of include:

• Skin reactions at application sites (for gels or patches)

• Acne or oily skin

• Fluid retention

• Breast tenderness (gynaecomastia)

• Increased red blood cell production, requiring monitoring (treatment may need adjustment if haematocrit exceeds 0.54)

• Potential effects on fertility, as TRT can suppress sperm production

• Possible sleep apnoea exacerbation

• Reduced testicular volume

If you are trying to conceive, TRT should be avoided as it suppresses sperm production. Alternative treatments may be discussed with a specialist.

NICE guidance recommends regular monitoring during treatment, including testosterone levels at 3 and 6 months, then annually, alongside PSA, haematocrit, and clinical assessment. Men on TRT should never expect penile enlargement as an outcome. The treatment aims to restore testosterone to normal physiological levels, thereby improving symptoms of deficiency. Supraphysiological doses (above normal levels) do not provide additional benefits for sexual function and significantly increase the risk of adverse effects.

If you experience any suspected side effects, report them via the MHRA Yellow Card scheme.

Speaking to Your GP About Concerns

If you have concerns about penile size or sexual function, it is important to have an open, honest conversation with your GP. Healthcare professionals are accustomed to discussing sensitive topics and can provide evidence-based guidance tailored to your individual circumstances.

When to contact your GP:

• You are experiencing symptoms of low testosterone (reduced libido, erectile dysfunction, persistent fatigue, mood changes)

• You have concerns about genital development (particularly relevant for adolescents or parents of adolescent boys)

• You are experiencing sexual dysfunction affecting your quality of life or relationships

• You have been considering testosterone treatment and want to understand if it is appropriate for you

• You notice any concerning symptoms such as new penile curvature, painful erections, prolonged erections (priapism lasting >4 hours requires urgent medical attention), testicular lumps, or breast changes

What your GP will assess includes a detailed medical history, including sexual health, general wellbeing, and any relevant medical conditions. They will perform a physical examination if appropriate and arrange blood tests to measure testosterone levels (typically two morning samples). Your GP will also explore psychological factors, as concerns about penile size can sometimes reflect body dysmorphia or anxiety rather than a medical issue.

Important considerations include understanding that there is significant normal variation in penile size, and the vast majority of men who have concerns actually fall within the normal range (average adult stretched penile length is approximately 13 cm). Micropenis, defined as an adult stretched length of less than 9-10 cm, is rare and requires specialist assessment. Testosterone therapy is not appropriate as a cosmetic treatment and should only be prescribed for genuine medical indications.

Your GP can refer you to appropriate specialists if needed, including:

• Endocrinology for hormone disorders (paediatric endocrinology for adolescents)

• Urology or andrology for structural concerns, micropenis, or sexual dysfunction

• Psychosexual medicine for psychological aspects of sexual health

• Psychology or counselling services if body image concerns are affecting mental health

For erectile dysfunction, first-line treatments typically include PDE5 inhibitors (such as sildenafil) rather than testosterone, unless true hypogonadism is present.

Patient safety advice: Be wary of unregulated online sources offering testosterone or other treatments for penile enlargement. These products may be counterfeit, contaminated, or contain undeclared substances. Unsupervised testosterone use carries significant health risks, including cardiovascular complications, infertility, liver damage, and psychological effects. Always seek treatment through regulated NHS or private healthcare providers where proper monitoring and follow-up can be ensured.

Frequently Asked Questions