Like all medicines, letrozole can cause side effects, although not everyone gets them.
Common side effects
Letrozole can cause symptoms similar to the menopause such as hot flushes, difficulty sleeping, tiredness and low mood, but these usually improve during the first months of taking it. However, if the symptoms are severe or last longer than a few months, talk to your doctor or breast cancer nurse.
These common side effects of letrozole happen in more than 1 in 100 people. There are things you can do to help cope with them:
Hot flushes and sweating
Try cutting down on spicy food, caffeine, smoking and alcohol. It may help to keep the room cool and use a fan. Try spraying your face with cool water, or sipping a cold drink.
Bleeding from your vagina
Vaginal bleeding usually happens in the first few weeks after starting letrozole. Talk to your doctor if it lasts longer than a few days.
Also talk to your doctor if vaginal bleeding first appears more than a few weeks after you start taking letrozole.
Feeling very tired
Gentle exercise and eating a healthy diet can help make you feel less tired. Try going to bed and getting up at a set time each day.
Do not drive, ride a bike or operate machinery if you feel very tired while taking letrozole.
Speak to your doctor if this problem does not gradually improve as your body gets used to the medicine.
Feeling or being sick
Stick to simple meals and do not eat rich or spicy food. It might help to take letrozole after you've eaten.
If you're being sick, try small frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark strong-smelling pee.
If you take contraceptive pills and you're being sick your contraception may not protect you from pregnancy. Check the pill packet for advice.
Talk to your doctor if your symptoms do not improve or get worse.
Losing your appetite
It might help to take letrozole after you've eaten.
Eat when you would usually expect to be hungry. If it helps, eat smaller meals more often than usual. Snack when you're hungry. Have nutritious snacks that are high in calories and protein, such as dried fruit and nuts.
Mild aches in your muscles or bones
Ask a pharmacist to recommend a suitable painkiller. If the pain does not go away ask your doctor for advice.
If you have a sudden attack of pain in a joint ask a doctor for advice urgently. Letrozole can make your bones weaker and there is an increased risk of osteoporosis or bone injuries.
Hair loss
Some people find their hair gets thinner when they start taking letrozole. This is usually mild. Ask your breast cancer nurse for advice if this is bothering you.
Low mood or depression
It's difficult to know whether this is due to the medicine, dealing with menopausal symptoms or dealing with a diagnosis of cancer.
Speak to your doctor or specialist nurse. They may recommend therapies, like cognitive behavioural therapy or mindfulness, or antidepressant medicines.
Talk to your doctor or a pharmacist if this advice on how to cope does not help and these side effects bother you or do not go away.
Serious side effects
Serious side effects are rare and happen in less than 1 in 100 people.
Call a doctor or call 111 now if you have:
- throbbing or cramping pain, swelling, warmth and redness (this may be harder to see on brown or black skin) in a leg or arm – these can be signs of a blood clot
- muscle weakness, pain or swelling in the joints or tendons in your ankle, calf or legs, shoulders or arms
- a high temperature or chills – these can be signs of an infection
- frequent mouth ulcers or frequent sore throat – these can be signs of a low white blood cell count
- yellow skin (this may be less obvious on brown or black skin) or the whites of your eyes turn yellow – this can be a sign of liver problems
- problems when you pee, including peeing more often or urgent need to pee – these can be signs of a urinary tract infection (UTI)
- blurred vision
Immediate action required: Call 999 or go to A&E if:
- you have sudden weakness or if you lose feeling in any part of your body, if your face drops on 1 side or you have difficulty speaking – these can be signs of a stroke
- you have very sudden chest pain – this can be a sign of a heart attack
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to letrozole.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Long-term side effects
Letrozole is generally safe to take for a long time. However, it can make your bones weaker and more likely to break (osteoporosis).
You may have bone density (DEXA) scans to check how strong your bones are, before you start taking letrozole, 1 or 2 years into treatment and again after you finish your treatment.
Your doctor may recommend a type of medicine called a bisphosphonate to help make your bones stronger. They can also give you advice on exercise and diet to help keep your bones strong.
Letrozole can also increase your blood pressure and cholesterol levels. Your doctor will monitor these during your regular check-ups and recommend treatment if you need it.
Other side effects
These are not all the side effects of letrozole. For a full list, see the leaflet inside your medicines packet.
You can report any suspected side effect using the Yellow Card safety scheme.