Testosterone

Key facts

• Testosterone is the principal androgen (male sex hormone). Prescription testosterone, as a gel, injection, patch or implant, treats hypogonadism: low testosterone confirmed on repeated blood tests with symptoms such as fatigue, low libido or muscle loss.
• Gels are applied once daily to clean, dry skin; injections are given every one to twelve weeks depending on the product. Benefits on energy, mood and libido build over weeks to months.
• Testosterone is contraindicated in pregnancy: it can permanently virilize a female fetus, causing irreversible masculinization of the genitals. With gel, skin contact can transfer the hormone to a partner or child, so wash your hands after applying it and keep the site covered once dry.
• Seek urgent care for chest pain, one-sided weakness or slurred speech, or a calf that is suddenly swollen, red and painful.

What testosterone treats

Testosterone replacement treats hypogonadism in men, low testosterone confirmed on repeated morning blood tests. It is not a treatment for normal age-related decline without confirmed deficiency, and it does not treat infertility; it can worsen it by suppressing sperm production. Some women are prescribed low-dose testosterone off-label for low sexual desire after menopause.

How testosterone works

Testosterone binds androgen receptors throughout the body, in muscle, bone, brain and reproductive tissue, switching on genes that maintain muscle mass, bone density, sperm production and libido. Replacement restores blood levels rather than fixing the underlying cause of low production.

Before you take it

• Do not use testosterone if you are pregnant, could become pregnant, or are breastfeeding: it will virilize a female fetus.
• Tell your prescriber about prostate or breast cancer, untreated severe sleep apnoea, uncontrolled heart failure, or a high red blood cell count. These usually rule it out.
• Testosterone can worsen an enlarged prostate and unmask undiagnosed prostate cancer, so older men need a baseline prostate check.
• If gel contacts another person's skin, wash the area with soap and water immediately.

Side effects

Common effects include acne, oily skin, fluid retention, mood changes and irritation at the application or injection site.

Stop and seek urgent medical care for any of these:

• Chest pain, shortness of breath, or one-sided weakness or numbness.
• Pain, redness or swelling in one leg.
• Worsening sleep apnoea, such as a partner noticing you stop breathing during sleep.
• Rapid weight gain or breathlessness.

Safety essentials

• Never use testosterone in pregnancy: it virilizes a female fetus, and the effect is irreversible. Women who could become pregnant should use effective contraception.
• With gels, wash your hands after each application and cover the site with clothing once dry; unwashed contact can raise hormone levels in a partner or masculinize a child.
• Regular monitoring is required: testosterone level, haematocrit (red blood cell volume) and PSA (a prostate marker) are checked before starting and periodically after. A rising haematocrit raises clot and stroke risk and may mean lowering or stopping the dose.
• Buy testosterone only from a licensed pharmacy; unregulated "testosterone boosters" sold online are a common source of contaminated product.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.