Hormone Therapy Side Effects: How to Cope With Tamoxifen and Aromatase Inhibitors

What hormone therapy is and why it is used

Many breast cancers grow in response to the hormone oestrogen. According to Cancer Research UK, around 80 out of every 100 breast cancers (around 80%) are oestrogen receptor positive, often written as ER positive or ER+. For these cancers, hormone therapy, also called endocrine therapy, works by reducing the amount of oestrogen in the body or by blocking its effect. This starves the cancer of the signal it uses to grow and helps stop the cancer coming back.

Hormone therapy is usually taken as a daily tablet after your main treatment, such as surgery, chemotherapy or radiotherapy, has finished. The two most common types are tamoxifen and a group of drugs called aromatase inhibitors. This guide explains the side effects each can cause and the practical things that help. It is general health information, not personal medical advice. Your breast care team, oncologist and GP know your situation and can tailor advice to you, so always speak to them about any symptoms or worries.

Tamoxifen and aromatase inhibitors: the difference

These medicines do the same job, lowering oestrogen's effect on breast cancer, but in different ways, and they suit different people. Cancer Research UK explains that the choice often depends on whether you have been through the menopause.

• Tamoxifen works by blocking oestrogen receptors on cancer cells, so oestrogen cannot reach them. It can be used by women who are pre-menopausal (still having periods) as well as those who are post-menopausal.
• Aromatase inhibitors, which include anastrozole, letrozole and exemestane, stop the body making oestrogen in fatty tissue. Cancer Research UK notes these are used by women who have been through the menopause.
• Some women take tamoxifen for a few years and then switch to an aromatase inhibitor, or use a treatment to switch off the ovaries alongside one of these tablets. Your team will explain the plan that is right for you.

Because they affect oestrogen in different ways, the side effects can differ too. Hot flushes, for example, are common with both, while joint and muscle aches are more linked to aromatase inhibitors, and a small increase in blood clot risk is linked to tamoxifen. Knowing which medicine you are on helps you and your team watch for the effects that matter most.

Why 5 to 10 years, and why it is worth it

Hormone therapy is taken for a long time, which can come as a surprise. Cancer Research UK says most people take tamoxifen for 5 years, and that hormone therapy is often given for 5 years or more, with some people taking it for up to 10 years, for example tamoxifen for 5 years followed by an aromatase inhibitor such as letrozole for a further 5. Your team decides the length based on your type of cancer, your risk of it returning and how you cope with the treatment.

The reason for the long course is that taking the medicine over several years gives the best protection against the cancer returning, sometimes many years later. Stopping early can reduce that protection. So although the side effects are real and can be wearing, the goal is firmly in your favour: keeping the cancer from coming back. If side effects make you think about stopping, please talk to your team first, as there are usually ways to make treatment easier rather than giving it up.

Hot flushes, sweats and mood changes

Hot flushes and night sweats are among the most common effects of hormone therapy, because the treatment lowers oestrogen and brings on menopause-like symptoms. The NHS notes that with tamoxifen these often ease over the first few months, though some people continue to have them for as long as they take the tablet. They are uncomfortable but not harmful.

Some practical steps can take the edge off. The NHS suggests that cutting down on triggers such as tea, coffee, alcohol and smoking can help reduce flushes for some people. Other things many people find useful include:

• Wearing layers of light, natural-fibre clothing you can easily take off, and keeping bedrooms cool with a fan.
• Sipping cold drinks and keeping a handheld fan or cool water spray nearby.
• Noticing your own triggers, such as spicy food or stress, and reducing them where you can.
• Asking your GP about options if flushes are severe, as some non-hormonal treatments can help. Avoid starting any new remedy without checking it is safe with your treatment.

Lower oestrogen, broken sleep and the emotional weight of cancer treatment can all affect mood. Feeling low, anxious or more tearful than usual is common and understandable. Gentle exercise, good sleep habits and staying connected with people you trust can help. If low mood lasts more than a couple of weeks or affects daily life, speak to your GP, who can offer support such as talking therapies.

Joint pain, stiffness and muscle aches

Aching joints and stiffness are a well-known effect of aromatase inhibitors. Studies referenced by UK cancer sources suggest joint symptoms affect a large share of women taking these drugs, sometimes more than half. The aches often affect the hands, wrists, knees and hips, and are frequently worse first thing in the morning before easing as you move about.

Reassuringly, this kind of joint pain does not mean lasting damage to your joints, and there is a lot you can do to ease it:

• Keep moving. Regular gentle activity such as walking, swimming or stretching helps keep joints supple and can reduce stiffness.
• Warm up slowly in the morning, and try a warm bath or shower to loosen stiff joints.
• Ask your GP or pharmacist about suitable pain relief, such as paracetamol, before reaching for anything new.
• Tell your team if the pain is severe or stopping you doing everyday things. They may suggest physiotherapy, a change of drug or a short break, and switching to a different aromatase inhibitor sometimes helps.

It is worth raising joint pain even if it feels minor, because it is one of the main reasons people are tempted to stop treatment. Small adjustments can make a real difference to staying the course.

Vaginal dryness, sex and intimacy

Lower oestrogen can cause vaginal dryness, itching or discomfort, and can affect interest in sex. Cancer Research UK notes that hormone treatments for breast cancer commonly cause vaginal changes such as dryness or discharge. These changes are common, nothing to be embarrassed about, and usually manageable.

UK guidance, including NHS hospital information, suggests starting with non-hormonal options. Vaginal moisturisers used regularly can ease dryness, and lubricants can help make sex more comfortable. A key point for breast cancer is that women on endocrine therapy for hormone-positive breast cancer are usually advised to avoid products containing oestrogen, such as some HRT and certain vaginal creams or pessaries, unless their specialist agrees. Always check with your breast team or GP before using any hormone-containing product. If dryness is severe or simple measures are not helping, your GP can discuss other options with you. Our guide on sex and intimacy after breast cancer covers this in more detail.

Looking after your bones

Because aromatase inhibitors lower oestrogen, which helps keep bones strong, they can cause bone thinning, known as osteoporosis. Cancer Research UK explains that this can sometimes lead to fractures. The good news is that this is monitored and treatable. You may be offered a DEXA scan, a quick and painless bone density scan, before or during treatment to check your bone strength, and it may be repeated to track any change.

There is plenty you can do to protect your bones:

• Do regular weight-bearing exercise such as walking, which Cancer Research UK notes helps build and protect bone.
• Make sure you get enough calcium and vitamin D; your team may suggest supplements.
• Stop smoking and keep alcohol within recommended limits, as both weaken bones.
• If your bone density is low, your team may prescribe bone-strengthening medicines called bisphosphonates, which can also help lower the risk of the cancer returning in some women.

Tamoxifen affects bones differently and is generally not linked to the same bone thinning in post-menopausal women. Our separate guide on bone health after breast cancer goes into more depth.

Sticking with treatment, and when to seek help

Taking a tablet every day for years, sometimes while coping with side effects, is genuinely hard. Yet keeping to your treatment, often called adherence, is one of the most important things you can do to lower the chance of the cancer returning. If you are struggling, the answer is rarely to stop on your own. Your team would much rather hear from you so they can help, whether that means treating a side effect, adjusting the dose, switching drugs or planning a review.

• Build the tablet into a daily routine, such as taking it at the same time each day, and use a pill organiser or phone reminder if that helps.
• Keep a simple note of side effects to share at appointments.
• Order repeat prescriptions in good time so you do not run out.
• Tell your team about side effects early, before they wear you down.

Some symptoms need prompt medical attention. The NHS advises that, because tamoxifen carries a small increased risk of blood clots, you should tell your doctor straight away if you have pain, warmth, swelling or tenderness in an arm or leg, or any chest pain or breathlessness. Also contact your GP or team about unusual vaginal bleeding, severe mood changes, or any new symptom that worries you. If you ever have sudden chest pain, severe breathlessness or signs of a serious clot, seek urgent medical help.

Hormone therapy can feel like a long road, but it is one of the most effective ways to protect against breast cancer returning. Clear information and good support make it easier to keep going. As a charity, we work to improve early detection and access to that kind of support in underserved communities, where reliable information and follow-up care can be hardest to reach. If anything in this guide raises questions for you, your breast care nurse, oncologist or GP is the best person to ask.