HRT, blood clots and stroke: why the route of oestrogen matters

When oestrogen is swallowed, it passes through the liver first, which changes the production of clotting factors and other proteins. This is thought to explain the small rise in clot risk. Oestrogen absorbed through the skin bypasses this first pass and has little effect on clotting factors at standard doses. That is why patches, gels and sprays are often preferred for women with higher baseline risk.

NICE guidance on menopause explains that the risk of venous thromboembolism is increased with oral oestrogen but not with transdermal oestrogen at standard doses. It also advises considering transdermal preparations rather than oral for women at higher risk of VTE, including those with a body mass index over thirty, though specialist advice may be needed for women with a personal history of clots. Risks of stroke show a similar pattern.

• A past DVT or pulmonary embolism, or a strong family history of clots.

• Known clotting disorders.

• Obesity and prolonged immobility, for example after surgery.

• Smoking, high blood pressure or migraine with aura, which are relevant to stroke risk.

• Planned surgery or long-haul travel, where preventive steps may be advised.

Evidence suggests that HRT started in women under sixty or within ten years of menopause is not linked to an increased risk of coronary heart disease, and may reduce it with oestrogen alone. Starting later may carry more risk. HRT is not prescribed to prevent heart disease. Women with heart disease should discuss it with a specialist.

Seek urgent help for a painful, swollen, red or warm leg, sudden breathlessness, chest pain, coughing blood, sudden severe headache, weakness or numbness on one side, difficulty speaking or sudden loss of vision. Call 999 for suspected stroke or pulmonary embolism. Tell clinicians that you take HRT.

Tell the surgical team about HRT. Depending on the operation, they may suggest stopping for a period or using preventive measures. For long-distance travel, move regularly, keep hydrated and consider compression stockings if advised. Do not stop HRT without advice for routine travel, since stopping carries its own consequences.

If you have any risk factors for clotting, it is reasonable to ask which route your prescriber recommends and why, whether you need any tests or specialist input first, what dose they would start with and how soon you will be reviewed. Ask what to do if you need an operation or a long period of bed rest. Ask whether any of your other medicines affect your risk. Bring a note of your family history, because relatives with clots at a young age can change the advice.