Tamoxifen

What it is

Tamoxifen is a selective oestrogen receptor modulator: it blocks oestrogen receptors in some tissues while acting like oestrogen in others. It is one of the most important cancer medicines of the last fifty years. In a performance context it appears both to counter gynaecomastia and as part of attempts to restart natural testosterone production after steroid use.

In oncology it is prescribed for hormone-receptor-positive breast cancer and for risk reduction in people at high genetic risk, with regular review and monitoring for its recognised complications.

What’s claimed

• Reversal or prevention of gynaecomastia
• Restarting the body's own testosterone production after a cycle
• Fewer side effects than an aromatase inhibitor

What the evidence shows

Tamoxifen's action on the oestrogen receptor is well-supported (Gold), and it does raise gonadotrophins and testosterone in men: the mechanism behind its use in post-cycle protocols is real. What is not established is that any self-directed protocol reliably restores the hormonal axis; the evidence for that is largely anecdotal, and recovery from suppression varies enormously between people and may be incomplete regardless of what is taken.

The risks

• Thrombotic: an established increase in the risk of venous thromboembolism, including deep-vein thrombosis and pulmonary embolism.
• Ocular: retinal changes and cataract have been reported with prolonged use.
• Hepatic: liver enzyme abnormalities and, rarely, more serious liver effects.
• Vasomotor and mood: hot flushes, low mood and reduced libido are common.
• Prescription-only: outside a prescription there is no assessment of thrombotic risk, which is the complication that most often does serious harm.

What needs monitoring

• Full blood count and liver function
• Testosterone, LH and FSH where hormonal recovery is the concern
• Any leg swelling, chest pain or breathlessness needs urgent medical attention rather than self-management.