Side effects of letrozole (Femara)

Like any drug, letrozole can cause side effects. Everyone reacts differently to drugs and some people have more side effects than others. 

For many people, side effects improve within the first few months of starting the treatment. However, for some people they may cause distress and disrupt everyday life.  

If you have any side effects, whether they’re listed here or not, talk to your GP or treatment team. They may be able to offer ways of improving your symptoms. 

Your specialist may suggest changing to a different aromatase inhibitor such as anastrozole or exemestane, or another hormone treatment, as some women get on better with a different drug.  

Letrozole is made by a number of different manufacturers. Some of the tablets may have different additional ingredients (for example preservatives). This does not change the effectiveness of the treatment. Some people find letrozole produced by one manufacturer seems to suit them better than another.

You may wish to discuss this with your treatment team or GP, or ask the pharmacist who dispenses your prescriptions if they can supply you with letrozole from the manufacturer you feel suits you the best.

Aching or pain in the joints and muscles 

Aching or pain in the joints and muscles is often mild and temporary, although for some people it’s more severe and longer lasting.

It can usually be relieved with mild pain relief like paracetamol or an anti-inflammatory drug such as ibuprofen. Before using anti-inflammatory pain relief, ask your doctor about the correct dose, how long you should use it for and any possible side effects, especially if you have a stomach ulcer or asthma. 

Exercise that gently stretches your joints or strengthens your muscles to better support your joints, such as yoga or Pilates, may help improve your symptoms. Brisk walking can also be beneficial. A physiotherapist may also be able to suggest exercises to help. If you have secondary breast cancer, check with your specialist before starting any new type of exercise.

Some people benefit from switching to a different hormone therapy. For example, it may be possible to switch to another aromatase inhibitor to see if your pain improves, or some people might be offered the drug tamoxifen as an alternative. 

If the pain is severe, it may be helpful to see a pain management specialist at a pain clinic.

Menopausal symptoms 

You may have menopausal symptoms such as hot flushes, sweating, sleep disturbance, vaginal irritation such as dryness and itching, and reduced libido (sex drive). Many people find that symptoms improve over time. 

Find out more about menopausal symptoms or sex and intimacy.

Low mood and depression 

Many people feel low or depressed. It can be difficult to know whether this is because of the medication or other reasons such as menopausal symptoms or dealing with the diagnosis of cancer. 

Talk to your GP or treatment team about how to manage a change in mood. Some people find counselling or mindfulness helpful. Others find exercise improves their mood. Your GP or specialist can refer you for counselling or may suggest you take an antidepressant drug. 

Your breast care nurse may also be able to offer support or tell you about support services in your area. 

Difficulty sleeping 

If you have difficulty sleeping (insomnia), some simple things like limiting caffeine in the afternoon and evening, keeping your room dark and quiet, and going to bed and getting up at a set time each day may help. 

Relaxation exercises can also be helpful. There are CDs, podcasts and phone apps that can guide you through these techniques. 

If you continue to have difficulty sleeping your GP may prescribe something to help you sleep.

Extreme tiredness

Cancer-related fatigue is extreme tiredness that doesn’t go away with rest or sleep. It’s a very common side effect of breast cancer treatment and may affect you physically and emotionally. If you think you have fatigue, tell your GP or treatment team. They can assess you and offer advice on how to manage your energy levels.

Osteoporosis (thinning of the bone) 

Letrozole reduces the amount of oestrogen in the body. A lack of oestrogen over time can cause osteoporosis. 

Your specialist will sometimes check the strength and thickness (density) of your bone with a scan before or shortly after you start taking letrozole. If you have been given bisphosphonates as part of your treatment you will not need a scan.

Your bone density may be checked every two to three years while you’re taking letrozole, depending on the results from the initial scan or whether your specialist has any concerns. 

To help keep your bones healthy you can increase your intake of calcium and vitamin D. Most people do this by taking a supplement. You may also want to eat more foods that are rich in calcium, such as milk, cheese, yoghurt and cereals. For vitamin D, you can increase the amount of oily fish, mushrooms and eggs you eat.

Stopping smoking and doing regular weight-bearing exercise can also help keep your bones strong. 

If your bones are already beginning to show signs of thinning or if you already have osteoporosis, you may be given another drug to increase bone density and strength. This will usually be from a group of drugs called bisphosphonates. If you’re concerned about this you may find it helpful to speak to your specialist. 

Find out more about looking after your bones and osteoporosis.

High blood pressure (hypertension) and high cholesterol 

Letrozole can cause high blood pressure (hypertension) in some people. Your GP will be able to monitor and treat this if necessary. 

It may also cause the level of cholesterol in the blood to rise, although this doesn’t usually need treatment. If you have a history of high cholesterol you may want to discuss this with your specialist or GP.

Headaches

Letrozole can sometimes cause headaches. Mild pain relief such as paracetamol may help with headaches.

Dizziness 

Letrozole may cause dizziness. If you feel dizzy, avoid driving. If dizziness persists see your GP. 

Nausea and vomiting

Letrozole may also make you feel sick (nausea) or be sick (vomiting).

These symptoms usually improve or become easier to manage over time. Taking letrozole with or after food can help reduce nausea.

Diarrhoea and constipation

If you have diarrhoea, it’s important to drink enough fluids. Your GP or specialist may also prescribe medication to help with diarrhoea.

It’s also important to keep active and eat a high-fibre diet to avoid constipation.

Change to your appetite 

If you lose your appetite, it may help to take your tablet with food. You may find eating small frequent meals or snacks helps keep up your food intake.  

If your appetite increases it may affect your weight. 

If you struggle to maintain a healthy weight, ask your GP or specialist about being referred to a dietitian. 

Find out more about diet and breast cancer treatment.

Changes to hair or facial hair

You may have some hair loss or thinning while taking letrozole. A small number of women notice an increase in downy facial hair.

Skin changes

You may develop a skin rash while taking letrozole. This usually goes away when your treatment finishes. Your pharmacist, GP or treatment team can tell you what products you can use on your skin to help.

Vaginal bleeding 

Vaginal bleeding can happen in the first few weeks after starting letrozole. It most commonly occurs when changing from one hormone therapy to another. This may also be due to vaginal dryness. If the bleeding continues for more than a few days, tell your specialist. 

Fast heart rate (palpitations) and chest pain

You may feel like your heart is racing (heart palpitations) when taking letrozole, which is more common in older people. Some people also experience chest pain. Speak to your GP or treatment team if you have any of these symptoms.

Swelling of arms, hands, feet and ankles (oedema)

You can get swollen arms, hands, feet and ankles. If you notice this or any other swelling, let your GP know.

Carpal tunnel syndrome 

Carpal tunnel syndrome (CTS) is pressure on a nerve in your wrist. This causes pain, tingling and numbness in your arm, hand or fingers. Using a wrist splint can help. Sometimes it can go away on its own, but it can take months to get better. Speak to your GP or specialist if you have any of these symptoms.

Blood clots

People with breast cancer have a higher risk of blood clots such as a deep vein thrombosis (DVT). Their risk is higher because of the cancer itself and some treatments for breast cancer. If the cancer has spread to other parts of the body (secondary breast cancer), this also increases the risk.

People with a DVT are at risk of developing a pulmonary embolism (PE). This is when part of the blood clot breaks away and travels to the lung.

Blood clots can be harmful but are treatable so it’s important to report symptoms as soon as possible.

Side effects caused by letrozole will be different for each person. You may find your side effects reduce after stopping letrozole, for others they may continue after treatment finishes.

You may also experience side effects caused by other treatments or individual circumstance, such as the menopause. If you're worried about any long term side effects after stopping letrozole, speak to your treatment team or GP.