Testogel contains unmodified testosterone, identical to the hormone naturally produced by the human body. This distinguishes it from injectable testosterone preparations, which use chemically modified testosterone esters such as enanthate or cypionate. Available in the UK as 1% gel (50 mg in 5 g sachets) and 16.2 mg/g gel (metered-dose pump or sachets), Testogel delivers testosterone transdermally through the skin. The hydroalcoholic gel formulation allows steady absorption over several hours, providing more stable day-to-day testosterone levels compared to depot injections. Licensed by the MHRA for testosterone replacement therapy in adult males with hypogonadism, Testogel offers a physiological approach to hormone replacement.

What Type of Testosterone Is in Testogel?

Testogel contains unmodified testosterone that is identical to the hormone naturally produced by the human body. In the UK, Testogel is available in two formulations:

• Testogel 1% gel (50 mg in 5 g sachets): Each gram contains 10 mg of testosterone

• Testogel 16.2 mg/g gel (available as a metered-dose pump or sachets): Each actuation or sachet delivers a specific dose according to the product instructions

This distinguishes Testogel from injectable testosterone preparations, which typically use testosterone esters such as testosterone enanthate or testosterone cypionate. These esters require chemical modification to extend their duration of action when administered intramuscularly. In contrast, Testogel delivers unmodified testosterone transdermally, allowing for steady absorption through the skin.

The gel is manufactured as a clear or slightly opalescent hydroalcoholic preparation. The alcohol base facilitates rapid drying on the skin and aids testosterone penetration through the stratum corneum. Once applied to clean, dry skin (on the shoulders and upper arms for the 16.2 mg/g formulation; shoulders, upper arms or abdomen for the 1% formulation), the testosterone is gradually released into the bloodstream over several hours, providing more stable day-to-day levels compared to depot injections.

The Medicines and Healthcare products Regulatory Agency (MHRA) has licensed Testogel for testosterone replacement therapy in adult males with conditions associated with primary or secondary hypogonadism. Understanding the specific type of testosterone in Testogel helps patients and clinicians make informed decisions about the most appropriate formulation for individual circumstances.

How Testosterone in Testogel Works in the Body

Once applied to the skin, the testosterone in Testogel penetrates the epidermis and enters the systemic circulation via dermal capillaries. Unlike oral testosterone preparations, which undergo extensive first-pass hepatic metabolism, transdermal testosterone bypasses the gastrointestinal tract and liver initially, resulting in more predictable bioavailability.

After absorption, testosterone circulates in the bloodstream, where approximately 98% binds to plasma proteins—primarily sex hormone-binding globulin (SHBG) and albumin. The remaining 2% exists as free, biologically active testosterone, which can enter target cells throughout the body. Within these cells, testosterone exerts its effects through two primary mechanisms:

• Direct action: Testosterone binds to androgen receptors in tissues such as muscle, bone, and the central nervous system, promoting protein synthesis, bone mineralisation, and neurocognitive function.

• Conversion to active metabolites: The enzyme 5α-reductase converts testosterone to dihydrotestosterone (DHT) in tissues like the prostate and skin, where DHT has more potent androgenic effects. Alternatively, the enzyme aromatase converts testosterone to oestradiol in adipose tissue and other sites, contributing to bone health and lipid metabolism.

Testogel's transdermal delivery produces relatively stable serum testosterone concentrations over 24 hours, avoiding the pronounced peaks and troughs associated with intramuscular depot injections. Steady-state testosterone levels are typically achieved within a few days of initiating treatment, though clinical benefits may take several weeks to become fully apparent.

Regular monitoring of serum testosterone levels ensures therapeutic concentrations are maintained within the local laboratory's normal adult male reference range. The timing of blood tests varies by formulation: for Testogel 1%, measurements are often taken in the morning before gel application, while for Testogel 16.2 mg/g, sampling is typically done 2-4 hours after application at steady state, as specified in the product information.

Testogel Formulation and Absorption

The pharmaceutical formulation of Testogel is carefully designed to optimise transdermal testosterone delivery whilst maintaining patient acceptability and safety. The gel contains testosterone dissolved in an alcohol-based vehicle (typically ethanol), along with excipients including carbomer (a gelling agent), isopropyl myristate (a penetration enhancer), and sodium hydroxide for pH adjustment. The alcohol content serves multiple functions: it acts as a solvent for testosterone, facilitates rapid drying after application, and enhances skin permeability.

Following application to intact skin, the alcohol evaporates within minutes, leaving a thin film containing testosterone on the skin surface. The testosterone then gradually penetrates through the stratum corneum—the outermost layer of the epidermis—via passive diffusion. This process continues for several hours, creating a depot effect in the skin that sustains absorption throughout the day. For Testogel 1%, approximately 9–14% of the applied testosterone dose is absorbed systemically, with individual variation depending on factors such as skin thickness, temperature, and application site.

The recommended application sites vary by formulation:

• Testogel 1%: Shoulders, upper arms, or abdomen

• Testogel 16.2 mg/g: Shoulders and upper arms only

Patients should avoid applying Testogel to the genital area due to high absorption rates and potential local irritation. The gel should be spread thinly over a large surface area rather than concentrated in one spot, as this maximises the skin surface available for absorption.

Important application considerations include:

• Allow the gel to dry completely (3–5 minutes) before dressing

• Avoid washing the application site for at least 6 hours (for Testogel 1%) or as directed in the product information for Testogel 16.2 mg/g

• Cover the application area with clothing to prevent transfer to others

• Wash hands thoroughly immediately after application

• The gel is flammable until dry—avoid open flames, smoking, or heat sources during and just after application

Skin-to-skin contact with others should be avoided until the gel has dried and the area is covered, as testosterone can transfer to partners or children through direct contact, potentially causing virilisation in women or premature sexual development in children. Particular care should be taken to avoid contact with pregnant women. If contact occurs, the affected person should wash the area thoroughly with soap and water. If close skin contact is anticipated, patients should wash the application site beforehand.

Comparing Testosterone Types in UK Treatments

In the UK, several testosterone formulations are available for hormone replacement therapy, each with distinct pharmacological characteristics, administration routes, and clinical applications. Understanding these differences helps clinicians and patients select the most appropriate treatment option.

Transdermal preparations include:

• Testogel (1% and 16.2 mg/g formulations)

• Tostran (2% gel)

• Testavan (20 mg/g gel)

These deliver unmodified testosterone through the skin. They offer the advantage of daily dosing flexibility, ease of discontinuation if adverse effects occur, and avoidance of needles. However, they require consistent daily application, carry a risk of transfer to others, and may cause local skin reactions in some patients.

Intramuscular testosterone esters (such as Sustanon 250 or testosterone enanthate) are administered by deep intramuscular injection every 2–4 weeks. These depot formulations contain testosterone chemically modified with ester groups (propionate, phenylpropionate, isocaproate, or decanoate), which slow the release of testosterone from the injection site. Whilst injections eliminate concerns about skin transfer and may improve treatment adherence, they produce more pronounced fluctuations in serum testosterone levels, with peaks shortly after injection and potential troughs before the next dose. They may also be associated with a higher risk of erythrocytosis than transdermal preparations.

Testosterone undecanoate (Nebido) is a long-acting intramuscular preparation administered every 10–14 weeks following a loading phase. This formulation provides the longest duration of action but requires careful monitoring during dose initiation and offers less flexibility for dose adjustment.

NICE Clinical Knowledge Summaries do not mandate a specific testosterone formulation, instead recommending individualised treatment selection based on patient preference, lifestyle factors, comorbidities, and cost-effectiveness considerations. Transdermal preparations are often preferred for initial therapy due to their physiological delivery profile and ease of dose titration. Patients who struggle with daily adherence or have concerns about skin transfer may benefit from longer-acting injectable formulations.

Who Can Use Testogel and When It's Prescribed

Testogel is licensed in the UK for testosterone replacement therapy in adult males with hypogonadism—a condition characterised by insufficient testosterone production. Hypogonadism may be classified as primary (testicular failure) or secondary (hypothalamic-pituitary dysfunction), with various underlying causes including:

• Congenital conditions (Klinefelter syndrome, cryptorchidism)

• Acquired testicular damage (trauma, infection, chemotherapy)

• Pituitary or hypothalamic disorders (tumours, hyperprolactinaemia)

• Age-related testosterone decline (late-onset hypogonadism)

• Chronic systemic illnesses (obesity, type 2 diabetes, chronic kidney disease)

Before prescribing Testogel, clinicians must confirm biochemical hypogonadism through two separate morning serum testosterone measurements showing levels below the local laboratory's reference range. Borderline results (typically 8-12 nmol/L) may warrant assessment of sex hormone-binding globulin (SHBG) and calculated free testosterone. Biochemical findings should be accompanied by consistent clinical symptoms such as reduced libido, erectile dysfunction, fatigue, decreased muscle mass, mood disturbance, or reduced bone mineral density. UK guidelines emphasise that testosterone therapy should not be initiated based on symptoms alone without biochemical confirmation.

Absolute contraindications to Testogel include:

• Prostate cancer or male breast cancer (current or suspected)

• Hypersensitivity to testosterone or gel excipients

Cautions and relative contraindications include:

• Severe cardiac, hepatic or renal impairment

• Untreated obstructive sleep apnoea

• Haematocrit >54% (therapy should be paused and investigated)

Patients initiating Testogel require baseline investigations including prostate-specific antigen (PSA), digital rectal examination, full blood count, liver function tests, and lipid profile. Follow-up monitoring should occur at 3 and 6 months, then annually, assessing serum testosterone levels, haematocrit, PSA, and clinical response. Dose adjustments are made based on serum testosterone concentrations measured at the appropriate time for the specific formulation.

Importantly, testosterone therapy suppresses spermatogenesis and is not a fertility treatment. Patients should use contraception if pregnancy is possible with their partner.

Referral to an endocrinologist should be considered if there are signs of pituitary disease (headache, visual field defects), low/normal gonadotrophins with low testosterone, hyperprolactinaemia, or gynaecomastia.

Patients should contact their GP if they experience:

• Signs of prostate problems (urinary hesitancy, nocturia, weak stream)

• Symptoms of polycythaemia (headaches, dizziness, visual disturbances)

• Mood changes, aggression, or sleep disturbance

• Ankle swelling or breathing difficulties

• Skin reactions at application sites

Suspected adverse effects can be reported via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk).

Testosterone therapy is not appropriate for men with normal testosterone levels seeking performance enhancement or anti-ageing benefits, as there is no evidence supporting such use and potential cardiovascular and haematological risks exist.

Frequently Asked Questions