Risks and Side Effects
Feminizing hormones are widely prescribed and, for most people, low risk when they're monitored. Like any medicine they carry some risks, and a few factors, above all smoking, age and the form of estrogen, shape them most. Most of those are things you and your prescriber can manage.
I'm not a doctor and I've never taken hormones. I coordinate trans patients through surgery in Bangkok, where hormones come up in every pre-operative conversation, mostly about the first section below. This is general information, not a substitute for a prescriber who knows your history; don't stop or change your hormones because of anything here without talking to them.
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Blood clots
The most important serious risk of estrogen is a blood clot. Most often it's a deep vein thrombosis in a leg, which can break off and lodge in the lungs as a pulmonary embolism. Estrogen raises the risk because it affects the clotting proteins the liver makes.
Several things push the risk up: smoking above all, older age, being overweight, a previous clot or a family history of clots, long periods without moving (long flights, illness, surgery), and some inherited clotting conditions. Several push it down: stopping smoking, and the form of estrogen.
Route matters. Tablets raise clot risk more than transdermal patches or gel, so prescribers often move people at higher risk across. Ethinylestradiol, the estrogen in contraceptive pills, carries a clearly higher risk and guidelines advise against it. Estrogen explains why the route makes the difference.
Level matters too. Estradiol well above the target range adds clot risk without adding feminization, which is one reason the blood tests exist and why more isn't better.
Around surgery the risk is higher again, which is why many surgeons have rules about hormones before an operation. Hormones and surgery covers that.
If you smoke or vape nicotine
Tell your prescriber honestly and ask what help they can offer to stop. It's the single biggest thing that changes your clot risk, and many surgeons will require you to have stopped anyway.
Anti-androgen side effects
Each anti-androgen brings its own.
Spironolactone makes the body hold on to potassium. High potassium is uncommon in healthy people with normal kidneys, but it can affect the heart, which is why it's checked in routine bloods. Tell your prescriber about kidney problems, other blood pressure medicines, and potassium supplements or salt substitutes. Spironolactone also makes you pass more urine and can lower blood pressure, causing light-headedness on standing.
Cyproterone acetate is linked to meningioma, a usually benign tumour of the lining of the brain, with risk rising at higher doses and over years of use; European regulators restricted it in 2020 and lower doses are now standard in most places. Meningiomas grow slowly, and persistent headaches, changes in vision, seizures or new weakness are the things to mention. Cyproterone also raises prolactin, can lower mood, and rarely affects the liver, so prolactin and liver tests come with it. Anti-androgens covers what to ask if you've been on an older, higher dose.
Bicalutamide can rarely cause serious liver injury, so liver tests are needed, most closely in the first months.
GnRH agonists leave estrogen doing all the work of protecting bone, so too little estrogen alongside them means hot flushes, fatigue and, over time, bone loss.
Mood
For many people, hormones improve mood, sometimes dramatically. That isn't universal. Some people find their emotions more intense, and some find their mood drops, particularly on certain anti-androgens or progesterone, or when levels are too low or swinging a lot.
A persistent low mood is worth raising with your prescriber rather than riding out. There's often something to adjust. If you're having thoughts of harming yourself, get help that day: an emergency department, your doctor, or a crisis line where you live.
Prolactin, gallstones and other effects
Prolactin is a hormone from the pituitary gland that estrogen can raise, and cyproterone raises further. A mildly raised level is common and usually not a problem. A much higher level can occasionally point to a pituitary growth, so prescribers monitor it, and persistent headaches, changes in vision or nipple discharge should be mentioned.
Gallstones are more common on estrogen, as they are in cis women. Pain under the right ribs, especially after fatty food, is the classic sign.
Blood fats and blood pressure. Estrogen can raise triglycerides, and it can affect blood pressure in either direction depending on the regimen.
Breast cancer. Risk rises compared with cis men but appears to stay well below that of cis women. Breast awareness matters, and screening programmes and prescribers can advise when checks make sense for you. Long-term health covers screening, including the prostate, which is still there.
Monitoring is how most of this gets caught early. Expect bloods roughly every three months in the first year, then once or twice a year once stable: estradiol and testosterone, plus potassium and kidney function on spironolactone, prolactin and liver tests on cyproterone, liver tests on bicalutamide, and lipids. Blood tests and monitoring sets out the panel and schedule.
Fertility
Estrogen and anti-androgens reduce sperm production, often a lot, and the effect may not fully reverse even if you stop. If you might want biological children, the time to think about sperm storage is before you start. Fertility covers the options and what happens if you've already started.
Reduced fertility is not reliable contraception. If pregnancy is possible with a partner, don't rely on hormones to prevent it.
When to get help urgently
Get medical help the same day, through an emergency department or emergency number, if you have:
- Pain, swelling, warmth or redness in one leg, especially the calf
- Sudden shortness of breath, chest pain, or coughing blood
- A sudden severe headache, weakness or numbness on one side, drooping face, trouble speaking, or sudden change in vision
- Yellowing of the skin or eyes, or dark urine, particularly on bicalutamide or cyproterone
- Palpitations or unusual muscle weakness on spironolactone
Tell whoever sees you that you're on estrogen and what anti-androgen you take. It changes what they look for.
Once you've started
Keep a short note on your phone listing your hormones, forms and doses and who prescribes them. In an emergency, or with a new doctor, it's the first thing anyone needs, and it's hard to remember accurately under stress.
Frequently asked questions
What is the biggest risk of feminizing HRT?
Blood clots. Smoking, age and the form of estrogen change the risk the most, and absorbed-through-the-skin forms carry less than tablets. See blood clots.
Do I need to stop estrogen before surgery?
It depends on the surgeon and the operation. Some ask you to pause and many no longer do, especially on transdermal estrogen. See blood clots and the hormones and surgery guide.
Why does my prescriber check potassium?
Spironolactone stops the body clearing potassium, and high levels can affect the heart. It's rarely a problem in healthy people but it's why it's monitored. See anti-androgen side effects.
What symptoms need a doctor straight away?
Leg pain or swelling, chest pain or breathlessness, stroke symptoms, yellowing skin, or palpitations. See when to get help urgently.