Fulvestrant side effects: the complete list and timeline

Fulvestrant is a hormone treatment for hormone receptor positive breast cancer that has spread or come back, given as two injections into the buttock muscles. Many people find it one of the gentler cancer treatments, and they carry on with work, family life and daily routines while having it. The side effects people notice most often are soreness, bruising or a firm lump where the injections go, hot flushes, tiredness, feeling sick, headaches, and aches in the joints, muscles or back. These are usually mild to moderate. Blood tests may show a rise in liver enzymes, which is usually small and is watched at routine checks. Less common but more important problems include blood clots, bleeding at the injection site in people with a low platelet count or on blood thinners, nerve pain running down the leg from the injection area, and allergic reactions. Many effects first appear in the opening month, when the starting schedule adds an extra injection, and they often settle as the body adjusts. When fulvestrant is combined with another medicine, such as a CDK4/6 inhibitor, alpelisib or capivasertib, the partner medicine often causes more side effects than fulvestrant itself, so it helps to know which effect belongs to which drug. Tell your team about anything new, however small, because most side effects can be eased once they are known.

Injection soreness is the most common effect

Aching, tenderness or a small hard area in the buttock for a day or two after each visit is expected and usually eases on its own.

Most other effects are mild

Hot flushes, tiredness, nausea and joint aches are common but rarely make people stop treatment.

A few need prompt attention

A painful swollen leg, sudden breathlessness, heavy bleeding or yellowing of the skin should be reported the same day.

Side effect lists come from clinical trials and reports gathered over many years. They are useful, but they do not predict what will happen to you.

Some symptoms come from the cancer

Tiredness, bone aches, poor appetite and low mood can be caused by the cancer itself, by earlier treatment or by other health conditions. It is not always possible to say which cause is to blame.

Combinations change the picture

When fulvestrant is given with a tablet such as a CDK4/6 inhibitor, the tablet usually brings its own set of side effects, such as low white cell counts, loose motions or high blood sugar.

Side effects do not show the medicine is working

Having few side effects does not mean fulvestrant is failing. Scans, symptoms and sometimes blood markers show whether it is working.

What this page cannot tell you

It cannot tell you which side effects you will get or how strong they will be. Your team knows your health history and can advise you.

Fulvestrant suits most people with hormone receptor positive advanced breast cancer, but some situations call for extra checks or a change in approach.

Blood thinners or a low platelet count

Injections into muscle can bleed more in people taking medicines that thin the blood, or whose platelet count is low. Tell your team about every medicine you take, including aspirin.

Liver problems

Fulvestrant is processed by the liver. People with moderate liver problems need careful monitoring, and it is generally avoided when liver function is severely reduced.

Pregnancy and breastfeeding

Fulvestrant can harm a developing baby. Women who could become pregnant need reliable contraception during treatment and for a period afterwards, and should not breastfeed.