Hormone pills for fibroids, heavy periods or delaying periods: the risk of blood clots, DVT and stroke

Why this matters

Hormone medicines are often the first thing offered for fibroids, a bulky uterus, adenomyosis, heavy periods and irregular bleeding. Many women also use them to delay a period before a trip, wedding or exam. They can work well. But they are not all the same. A pill with estrogen, a progestin tablet and an injection that switches off the ovaries carry different risks. For many women, the risk that matters most is a blood clot.

This guide explains the differences in plain words: who should be most careful, which warning signs need urgent care, and what other options there are. It is general information and does not replace your own doctor's advice. Do not stop a medicine your doctor prescribed without talking to them first.

How the main medicines compare

The figures below come from large studies and trials. They describe groups of women. They cannot tell you exactly what will happen to you.

Pills with estrogen: how big is the clot and stroke risk?

Pills with estrogen make clots more likely, but for a healthy woman the chance is still small.

In a very large Danish study, about 3 in 10,000 women a year who did not use the pill had a clot in a vein. Among women on the pill, it was about 6 in 10,000. The risk was lower with lower estrogen doses, and lower the longer a woman had used the pill. Pills containing desogestrel, gestodene or drospirenone carried a higher risk than pills with levonorgestrel ^[2].

A UK study of more than 10,000 women who had a first clot found that women on combined pills were about 3 times as likely to have one. It was about 2.4 times for levonorgestrel pills and 2.6 times for norethisterone pills, but about 3.6 to 4.3 times for pills with desogestrel, gestodene, drospirenone or cyproterone ^[1].

Pills can also raise the chance of a stroke caused by a clot. In a Danish study of 1.6 million women, this kind of stroke happened in about 2 in 10,000 women a year overall. Pills with 30 to 40 micrograms of estrogen were linked to about 1.5 to 2.2 times that ^[3].

So the extra risk is real, but small for a healthy woman. It matters most when other risk factors are present (see below). That is why you should be checked before you start.

Norethisterone tablets to stop bleeding or delay periods

Norethisterone (also called norethindrone) tablets are one of the most common ways to delay a period or stop heavy bleeding. Because they are progestins, many people assume they are as safe as progestin-only pills. There is a catch.

In a study of women after menopause, the body turned a small part of norethisterone acetate into ethinylestradiol, the estrogen used in birth control pills. About 1 in 100 (0.7 to 1.0%) was changed this way. That works out to about 6 micrograms of ethinylestradiol for every milligram of norethisterone acetate ^[5]. At the 10 mg dose studied, that is roughly 60 micrograms, which is more than the 30 to 40 micrograms in many birth control pills ^[3]. This is a lab measurement. It does not tell us the actual clot risk for a woman taking the tablets.

How much these tablets raise the chance of a clot has not been measured as well as for birth control pills. Doctors have reported serious clots, including a clot in a vein of the brain in a young woman taking norethindrone acetate for abnormal bleeding ^[6]. Doctors have also called for safer prescribing for women at risk of clots ^[7].

In practice, short courses prescribed by a doctor are common, and many women take them without trouble. But they should be prescribed after your clot risk has been checked. They should not be bought over the counter and repeated month after month without a review.

GnRH injections and tablets: a different set of risks

GnRH medicines switch off the signal that tells the ovaries to make hormones. They do not contain estrogen. They stop periods and can shrink fibroids, but they can cause menopause-like effects such as hot flashes.

In a trial comparing ulipristal with a monthly leuprolide injection, bleeding was controlled in 89% of women on leuprolide. But about 4 in 10 of them (40%) had moderate to severe hot flashes, compared with about 1 in 10 on ulipristal ^[11].

Very low estrogen can also affect your bones. So newer treatments add back small amounts of estradiol and norethindrone acetate. In large trials, elagolix with these added hormones controlled heavy bleeding in about 7 out of 10 women (68.5% and 76.5%), compared with about 1 in 10 on dummy tablets ^[8]. Relugolix with added hormones worked for 71 to 73% of women, compared with 15 to 19% on dummy tablets, and it kept bone strength steady ^[9].

Because these combinations contain estrogen again, clots are a question. An expert review found that the trials and reports after launch had not shown a clear rise in clots with the relugolix combination, and that the risk looks lower than with birth control pills. It also said that women with clot risk factors must be chosen carefully ^[10].

Ulipristal has a different worry: liver injury has been reported ^[12]. Liver blood tests and careful selection matter.

Who should be especially careful

The CDC's 2024 guide to who can safely use which contraceptive method gives specific advice for women who have had a clot in the leg or lung, have a clotting disorder, are very overweight, or have had recent surgery ^[13]. Other things doctors commonly consider:

• You or a close relative has had a clot, especially at a young age
• Smoking, especially after age 35
• Being very overweight
• Not being able to move for a long time: after major surgery, in bed, or on a long journey
• Migraine with aura (flashing lights or blind spots before a headache)
• High blood pressure, diabetes or high cholesterol
• Older age, because clot risk rises with age

Warning signs that need emergency care

Go to the nearest emergency department, or call an ambulance, if any of these happen while you are taking hormone medicines, or in the weeks afterwards.

• A clot in the leg (DVT): pain, swelling, warmth or redness in one calf or thigh, usually on one side
• A clot in the lung: sudden breathlessness, chest pain, a fast heartbeat, dizziness, or coughing up blood
• A stroke: sudden weakness or numbness in the face, arm or leg, a drooping face, trouble speaking or understanding, sudden loss of vision, or a sudden, very bad headache

Safer ways to use hormones, and the alternatives

Hormones can be the right choice. These steps make them safer, and the alternatives are worth knowing about.

• Tell your doctor everything first: earlier clots, family history, smoking, migraine with aura, your weight, and any recent or planned surgery.
• Do not buy or repeat period-delaying tablets on your own. Ask for the lowest dose that works, for the shortest time.
• Before any planned operation, tell the surgeon and anaesthetist that you take hormones. On long journeys, walk about and drink water.
• If you need long-term control of heavy bleeding, ask about options with less estrogen. Progestin-only pills and hormone IUDs have not been linked to more clots ^{[2, 4]}.
• For fibroids, ask whether the cause can be treated, not only the bleeding. UFE blocks the blood supply to fibroids through a tiny puncture and keeps your uterus ^[15].
• For adenomyosis, a small trial of 60 women found that blocking the uterine arteries (UAE) relieved symptoms better than dienogest tablets after 6 months. It worked for about 87% of women, compared with 50%. This was one short trial in one hospital, so larger studies are needed ^[14].

If you already have a clot or leg swelling

A clot in the leg needs prompt help. A Doppler ultrasound is usually the first test. Treatment depends on where the clot is and how serious it is. Tell the doctor treating you if you take hormone medicines. Do not stop or restart them without medical advice, because the right decision depends on your whole situation. See our guides on DVT symptoms and treatment, and on clot removal.