The short answer
Hormone therapy blocks estrogen, usually for at least five years and sometimes up to ten. Hot flushes, joint pain and bone thinning can occur; tamoxifen adds clot, stroke and uterine risks. If you want to stop, talk with your oncologist first.
Tamoxifen blocks estrogen receptors and is used before and after menopause; aromatase inhibitors block estrogen production and are used mainly after menopause or with ovarian suppression.
Treatment is usually taken for at least five years, and sometimes up to ten.
Joint and muscle pain is a common aromatase inhibitor effect. Aromatase inhibitors can also thin bone, so ask your team about bone density monitoring.
In postmenopausal women tamoxifen raises the risk of uterine cancer, so call your care team right away about any unusual vaginal bleeding.
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The whole article, with every detail and caution.
This page is for people taking tamoxifen or an aromatase inhibitor for breast cancer. It covers common side effects, drug interactions, and what to report.
What these drugs are doing
Hormone therapy treats hormone receptor positive breast cancer. It works by cutting off estrogen signaling. Tamoxifen binds to estrogen receptors. That blocks estrogen's effect in breast tissue. NCI notes it is used both before and after menopause.
Aromatase inhibitors work a different way. They block the enzyme that makes estrogen. This group includes anastrozole, letrozole and exemestane. Doctors use them mainly after menopause. They can also be used before menopause, along with a drug that suppresses ovarian function. Treatment usually runs at least five years. Sometimes it runs up to ten.
What people commonly report
Hot flashes and night sweats top the list for both drug types. Vaginal dryness and reduced interest in sex are common too. With aromatase inhibitors, joint and muscle pain is common. ACS says it can feel like having arthritis in many joints at once. ACS lists a few options. One is switching to a different aromatase inhibitor. Others are regular exercise with anti-inflammatory medicines, a medicine such as duloxetine, or acupuncture. Aromatase inhibitors can also thin bone. Ask your team about bone density checks.
Tamoxifen carries a different set of risks. In women past menopause it raises the risk of cancer of the uterus lining. That is why any unusual vaginal bleeding needs prompt review rather than watchful waiting. It also raises the risk of blood clots in the legs and lungs. ACS says tamoxifen has been linked to strokes in women past menopause, although this is rare. Cataracts can sometimes develop. Before menopause it can cause some bone thinning. After menopause it may strengthen bone.
Drug interactions worth raising
Some antidepressants can make tamoxifen work less well. NCI says doctors may recommend sertraline, citalopram or venlafaxine instead. It also lists quinidine, diphenhydramine and cimetidine as drugs that could interfere. Diphenhydramine and cimetidine are sold over the counter. So tell your oncology pharmacist about everything you take. That includes antihistamines, heartburn remedies and supplements. If another clinician prescribes an antidepressant, mention the tamoxifen at that visit.
The part that is rarely said out loud
Taking a daily tablet for five to ten years is genuinely hard. It is harder when you feel worse than you did before it. ACS says the joint pain from aromatase inhibitors is sometimes severe enough that women stop treatment. You may be skipping doses, cutting tablets, or thinking about stopping. That is information your oncologist needs. It is not a failure to confess.
Concrete options often exist:
- switching to a different aromatase inhibitor
- switching to tamoxifen, which ACS says most doctors recommend if joint pain is severe on an aromatase inhibitor
- treating hot flashes or joint pain directly
None of this is on offer if the conversation never happens.
What to report, and when
Call 911 or go to an emergency department for sudden shortness of breath, chest pain, or coughing up blood. Those can mean a clot has reached the lung. Do the same for sudden severe headache, confusion, weakness or drooping on one side, or trouble speaking or seeing, which can be a stroke. Stroke care is timed in minutes, so do not drive yourself and do not wait for a call back from the clinic.
Also call 911 or go to the nearest emergency department for bleeding that will not stop, black or tarry stool, blood in vomit or vomit that looks like coffee grounds, or being hard to wake.
Call your care team right away for pain, redness, swelling or warmth in one calf. It may be a clot in the leg. Go to an emergency department if breathlessness or chest pain joins it.
Call your care team right away for any unusual vaginal bleeding or spotting, above all after menopause. ACS says most uterine bleeding is not cancer, but it always needs to be checked right away. Call right away for new vision changes, new pelvic pain or unusual discharge.
At routine visits, report joint or muscle pain, hot flashes that disrupt sleep, low mood and sexual side effects. Report any doses you have missed. Ask whether bone density checks apply to you. Ask whether calcium, vitamin D or a bone-protecting medicine applies to you.
Sources
Common questions
What is the difference between tamoxifen and an aromatase inhibitor?
Tamoxifen binds to estrogen receptors and blocks estrogen's effect in breast tissue, and NCI notes it is used both before and after menopause. Aromatase inhibitors, which include anastrozole, letrozole and exemestane, block the enzyme that makes estrogen. They are used mainly after menopause, or before menopause when combined with a drug that suppresses ovarian function.
Why do my joints hurt so much on an aromatase inhibitor?
Joint and muscle pain is a common effect of aromatase inhibitors. ACS says it can feel like having arthritis in many joints at once. ACS lists these options: switching to a different aromatase inhibitor, regular exercise with anti-inflammatory medicines, a medicine such as duloxetine, or acupuncture. Aromatase inhibitors can also thin bone, so ask your team about bone density checks.
Which medicines interact with tamoxifen?
NCI says some antidepressants can reduce how well tamoxifen works. NCI says doctors may recommend sertraline, citalopram or venlafaxine instead, and lists quinidine, diphenhydramine and cimetidine as drugs that could interfere. Because diphenhydramine and cimetidine are sold over the counter, tell your oncology pharmacist about everything you take, including antihistamines, heartburn remedies and supplements.
What bleeding or symptoms should I report right away?
Call your care team right away about any unusual vaginal bleeding or spotting on tamoxifen. ACS says most uterine bleeding is not cancer, but it always needs to be checked right away. Call 911 or go to an emergency department for sudden breathlessness, chest pain or coughing up blood, and for sudden severe headache, confusion, one-sided weakness or trouble speaking or seeing, since those can be a clot on the lung or a stroke. Call your care team right away for pain, swelling or redness in one calf, or any change in your vision.
Is it common to want to stop taking it?
Yes. Taking a daily tablet for years while feeling worse than before is genuinely hard. ACS says the joint pain from aromatase inhibitors is sometimes severe enough that women stop treatment. If you are skipping doses or considering stopping, tell your oncologist first. Options often exist, including switching to a different aromatase inhibitor or to tamoxifen, or treating the specific side effect.
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Written by: Cancer ExplainedSources last checked: 2026-07-30 what this meansLast updated: 2026-09-29Next planned review: 2027-01-30
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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