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Letrozole is a hormone therapy that lowers oestrogen levels. Read about when letrozole (Femara) is prescribed, how it works and potential side effects.

1. What is letrozole?

Letrozole is a hormone therapy drug used to treat breast cancer in women who've gone through a natural menopause (when periods stop). It can also be used in premenopausal women having .

You may also hear it called Femara, which is its best-known brand name. There are a number of other brands of letrozole, all of which contain the same dose of the drug. 

Men with breast cancer may be given letrozole, although another drug called tamoxifen is more commonly used.

2. How letrozole works

Letrozole works by reducing the amount of oestrogen made in the body. 

Some breast cancers use oestrogen in the body to help them to grow. These are known as oestrogen receptor positive or ER-positive breast cancers.

Letrozole is in a group of drugs called .

3. Who might be offered letrozole?

Letrozole is suitable for women who've been through the menopause and whose breast cancer is oestrogen receptor positive.

Sometimes letrozole is given alongside ovarian suppression to women who haven’t yet been through the menopause.

If your cancer is hormone receptor negative, then letrozole will not be of any benefit.

4. When is letrozole given?

Letrozole is usually given after surgery to reduce the risk of breast cancer coming back or spreading.

If you’re having or , your specialist will tell you when it’s best to start letrozole. 

Occasionally, letrozole may be used as the first treatment for breast cancer, for example when surgery isn’t appropriate or needs to be delayed. It’s sometimes given before surgery to shrink a larger breast cancer.

Letrozole can also be used to treat breast cancer that has come back (). It can also be given to treat breast cancer that has spread to another part of the body (), when it’s often given alongside another drug. 

5. How letrozole is given

Letrozole is taken as a tablet once a day, with or without food. It’s best to take it at the same time every day.

If you miss a dose, you don’t need to take an extra dose the next day. The level of drug in your body will remain high enough from the day before. Some brands of letrozole contain small amounts of lactose. If you know you are lactose intolerant discuss this with your treatment team or pharmacist.

6. How long will I have to take it? 

The recommended length of time that letrozole is taken for will depend on your individual situation.

People being treated for primary breast cancer will usually take letrozole for five to ten years.

Some people start taking letrozole after a few years of taking the hormone therapy drug

If you’re taking letrozole to treat breast cancer that has come back or spread to another part of the body, you’ll usually take it for as long as it’s keeping the cancer under control.

7. Side effects of letrozole

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

8. Stopping letrozole

Your treatment team will tell you when to stop taking letrozole. You won’t need to stop taking it gradually. 

Some people worry about stopping their treatment, but there’s evidence that letrozole continues to reduce the risk of breast cancer coming back for many years after you stop taking it.

However, not taking the drug for the recommended time may increase the risk of your breast cancer coming back. If you’re thinking about stopping taking letrozole for any reason, talk to your specialist first. Sometimes it may be possible to change to another hormone therapy.

Hormone therapy is a very common treatment for secondary breast cancer and many people take it for a long time. If letrozole stops working, your specialist may prescribe another hormone therapy drug. 

If you have any worries or questions about taking or stopping letrozole, you can call our free helpline on 0808 800 6000 to talk through your concerns.

9. Other important information

Taking letrozole with other drugs 

If you’re taking any other prescribed or over-the-counter medicines, check with your treatment team or pharmacist if you can take these with letrozole.

Do not take other drugs containing oestrogen, such as hormone replacement therapy (HRT), while you’re taking letrozole as this may interfere with its effectiveness. 

Talk to your specialist, pharmacist or GP about any complementary therapies, herbal remedies or supplements you want to use before you start using them.

Sex and contraception

Taking letrozole while pregnant may be harmful to a developing baby. It’s possible to become pregnant while taking letrozole even if your periods have stopped with ovarian suppression.

If you’re sexually active with a chance of becoming pregnant, your specialist is likely to advise you to use a non-hormonal method of contraception while taking letrozole. Non-hormonal methods include condoms, Femidoms or a diaphragm. It may also be possible to use a coil (IUD or intrauterine device). However, you would need to discuss this with your treatment team as not all types are suitable for women with breast cancer.

If you think you may be pregnant, stop letrozole and tell your specialist as soon as possible.


Getting pregnant while taking letrozole for breast cancer treatment is not recommended. It’s important to discuss any fertility concerns with your treatment team before you begin your treatment.


Breastfeeding is not recommended while having letrozole, or within a week of the last dose. This is because there’s a risk the drugs could be passed on through breast milk.

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Quality assurance

This information was published in April 2022. We will revise it in April 2024.

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