Medical Oncologist
Hormone therapy
Switching from tamoxifen to an aromatase inhibitor mid-course
Many women switch from tamoxifen to an aromatase inhibitor after reaching menopause, to gain a slightly larger benefit while completing their years of hormone therapy. Confirming menopause first is essential. This page explains, in general terms, why switches are made and what to expect.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
On this page
- Why would I switch from tamoxifen to an aromatase inhibitor?
- How a switch usually happens
- Why switches are made
- The vocabulary, in plain language
- What switching can and cannot do
- Why confirming menopause on tamoxifen is not simple
- Settling into your new tablet
- Questions to ask before switching tablets
- What people assume about switching tablets
- Common questions about switching
The short answer
Why would I switch from tamoxifen to an aromatase inhibitor?
The most common reason is that you have reached menopause while taking tamoxifen. After menopause, aromatase inhibitors lower the chance of recurrence slightly more than tamoxifen for most women. Studies have shown that switching after two to three years of tamoxifen to an aromatase inhibitor, to complete five years or more in total, works at least as well as staying on either tablet alone. Some switches are also made to ease side effects.
Why switching is planned in some cases
Starting with tamoxifen and changing later lets women who were premenopausal at diagnosis benefit from an aromatase inhibitor once menopause arrives. It also spreads the side effects of each drug across a shorter period.
What must be confirmed first
Your oncologist needs to be confident the ovaries have genuinely stopped working. Periods can stop on tamoxifen or after chemotherapy without menopause being complete, so blood tests are often used to check.
When switching may not be wise
If menopause is uncertain, if you have significant osteoporosis, or if tamoxifen is suiting you well with a lower-risk cancer, staying on tamoxifen can be a perfectly reasonable choice.
This page gives general information only. Any change in tablets is a decision for your oncologist.Step by step
How a switch usually happens
Reviewing your menopausal status
Your oncologist asks about periods and symptoms, and may arrange blood tests to check hormone levels, sometimes repeated over months.
A bone density scan
Because aromatase inhibitors can thin bones, a baseline scan is usually done before switching.
Stopping tamoxifen, starting the new tablet
The change is usually made directly, from one day to the next, without a gap.
Checking in after a few months
A review looks at side effects, especially joint pain, and repeats hormone tests if needed.
Continuing to the planned total
The aromatase inhibitor continues until you reach the total years of hormone therapy your oncologist recommends.
Reasons to switch
Why switches are made
Switches go in both directions and for different reasons.
Reaching menopause
The most common reason to move from tamoxifen to an aromatase inhibitor, to gain a slightly larger benefit.
Planned sequencing
Some plans deliberately combine a few years of each tablet from the outset.
Discussed at diagnosis.Tamoxifen side effects
Womb thickening, clot concerns or troublesome symptoms may prompt a change if menopause is confirmed.
Aromatase inhibitor side effects
Joint pain or bone loss sometimes leads to switching back to tamoxifen, or to a different aromatase inhibitor.
Before any switch
- Confirm menopause if moving to an aromatase inhibitor
- Check bone health
- Review other medicines
Words you will hear
The vocabulary, in plain language
- Switch strategy
- Taking tamoxifen for a few years and then an aromatase inhibitor.
- FSH and oestradiol
- Hormone blood tests used to help confirm menopause.
- Chemotherapy-induced amenorrhoea
- Periods stopping because of chemotherapy, which may or may not be permanent.
- Aromatase inhibitor
- A hormone tablet that lowers oestrogen after menopause.
- Bone density scan
- A scan measuring bone strength, often called a DEXA scan.
- Total duration
- The combined years of all hormone tablets taken.
Being straight with you
What switching can and cannot do
Switching to an aromatase inhibitor after menopause gives a modest additional benefit for many women. It is a refinement of treatment rather than a dramatic change, and staying on tamoxifen is still effective.
The biggest risk is uncertain menopause
If the ovaries restart while you are on an aromatase inhibitor, the tablet can lose much of its effect, and it may even stimulate the ovaries further. That is why careful checking matters, particularly for women in their forties or those whose periods stopped with chemotherapy.
New side effects can appear
Some women who managed well on tamoxifen find joint pain or stiffness on an aromatase inhibitor difficult. Tell your team early, because switching to a different aromatase inhibitor or back to tamoxifen is possible.
The total years still matter
A switch does not reset the clock. The aim is to complete the total years of hormone therapy that suit your cancer.
What this page cannot tell you
It cannot tell you whether switching is right for you. Ask your oncologist what the switch would add in your case.
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Knowing where you stand
Why confirming menopause on tamoxifen is not simple
Many women on tamoxifen are unsure whether they have reached menopause, and that uncertainty is completely normal.
Tamoxifen can blur the picture
Tamoxifen can make periods irregular or stop them, and it can affect hormone blood tests. Hot flushes happen on tamoxifen whether or not menopause has arrived, so symptoms alone are not a reliable guide.
Age helps but does not decide
Women in their late fifties with no periods for a long time are very likely postmenopausal. For women in their forties, especially after chemotherapy, more careful testing is needed.
Ovarian suppression as an alternative
When menopause cannot be confirmed, some oncologists add injections to switch off the ovaries, so an aromatase inhibitor can be used safely.
After switching
Settling into your new tablet
The first few months on a new tablet are when side effects are most likely to show. A little preparation makes the change smoother.
Protect your joints and bones
Keep active with regular walking, stretching or yoga, which often reduce joint stiffness. Ask whether calcium, vitamin D or a bone medicine is recommended for you.
Keep track of symptoms
Note any joint pain, mood changes or vaginal dryness in the first few months, and bring the notes to your review so problems can be addressed early.
Report any bleeding
Vaginal bleeding after switching can mean the ovaries have restarted or needs other checks. Tell your team promptly.
The conversation
Questions to ask before switching tablets
A switch is usually straightforward, but a short, clear conversation beforehand helps avoid surprises in the months that follow.
About your menopause
Ask how your oncologist has confirmed that you are postmenopausal, and whether repeat blood tests are planned after the switch. This matters most if you are in your forties or your periods stopped during chemotherapy.
About the benefit
Ask how much the switch is expected to add for your particular cancer, and what the plan would be if you preferred to stay on tamoxifen.
About side effects and bones
Ask which side effects to expect in the first months, what to do if joint pain appears, and whether you need a bone density scan, calcium, vitamin D or bone-strengthening treatment.
About the total length
Ask how many more years of hormone therapy are planned in total, so you know when the next review of the plan will be.
Commonly believed
What people assume about switching tablets
Switching means tamoxifen was not working.
Most switches are planned to take advantage of menopause or to ease side effects. They do not mean tamoxifen failed.
No periods on tamoxifen means I am postmenopausal.
Tamoxifen can stop periods without menopause being complete. Blood tests or ovarian suppression may be needed before switching.
After switching, I start counting the years from zero.
The years on tamoxifen count. The goal is the total years of hormone therapy your oncologist recommends.
If the new tablet hurts my joints, I should stop hormone therapy.
Joint pain can often be eased, and switching to another tablet is possible. Talk to your team before stopping.
Questions we are asked
Common questions about switching
When is the switch usually made?
Often after two to three years of tamoxifen, once menopause is confirmed. It can also happen later. Your oncologist will suggest the timing that suits you.
Do I need a gap between tablets?
No. The change is usually made from one day to the next.
Which aromatase inhibitor will I get?
Letrozole, anastrozole and exemestane work similarly. The choice may depend on availability, cost and side effects.
Can I switch back if I have problems?
Often yes. Switching back to tamoxifen or to another aromatase inhibitor is possible. Discuss it with your team.
Will I need bone scans?
Usually yes, before and during aromatase inhibitor treatment.
What if my periods return after switching?
Tell your oncologist promptly. The plan may need changing, for example back to tamoxifen or adding ovarian suppression.
Does switching change my follow-up?
Follow-up visits and mammograms continue as before, with added bone monitoring.
Where can I read about my new tablet?
Your oncology team will give you written information about your hormone therapy. Use that as your main reference.
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Sources
- National Cancer Institute — Hormone therapy for breast cancer
- Cancer Research UK — Hormone therapy for breast cancer
- American Society of Clinical Oncology — Adjuvant endocrine therapy for hormone receptor-positive breast cancer guideline
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