Abemaciclib side effects: the complete list and timeline

Abemaciclib is a CDK4/6 inhibitor tablet taken twice a day with hormone therapy for hormone receptor positive, HER2-negative breast cancer. Its most common side effect by far is diarrhoea, or loose motions. Most people who get it notice it within the first week or two, and for many it becomes milder after the first month or so, especially with anti-diarrhoea medicine and, when needed, a lower dose. The next most common effects are low blood counts, including a low white cell count, which raises the risk of infection, and a low haemoglobin, which can add to tiredness. Tiredness, feeling sick, tummy pain, reduced appetite, headache and thinning hair are also common. Blood tests often show a rise in liver enzymes, and sometimes a rise in creatinine, a marker usually used to check the kidneys. With abemaciclib that creatinine rise often reflects how the medicine affects the way the kidneys handle creatinine rather than true kidney damage, although your team will still check. Less common but important are blood clots in the legs or lungs and a rare inflammation of the lungs. Side effects vary widely: some people have very few, while others need dose pauses and reductions. The hormone partner medicine adds its own effects, such as hot flushes or joint aches. Nearly all of these are manageable when reported early.

Early weeks

Loose motions, feeling sick and tiredness tend to appear first, often within days of starting.

First two to four months

Blood counts and liver tests are checked often, because changes are most likely during this period.

Longer term

Many people settle into a steady pattern, with tiredness and hair thinning the effects that tend to linger.

Lists of side effects can be frightening. Most people have some, but rarely all.

Nobody can predict your own experience

Two people on the same dose can have very different side effects.

Hormone therapy adds its own effects

Hot flushes, joint aches and vaginal dryness often come from the partner medicine rather than from abemaciclib.

Rare problems are still possible

Clots and lung inflammation are uncommon, which is why new breathlessness or leg swelling should always be reported.

What this page cannot tell you

It cannot tell you which effects you will have or whether a symptom is from treatment. Your team can assess that.

Contact your oncology team straight away, day or night, if you have any of the following.

Signs of infection

A fever, shivering, a sore throat or feeling suddenly unwell.

Severe or ongoing loose motions

Especially with dizziness, very little urine or blood in the stool.

Clot or lung warning signs

A painful, swollen calf, sudden breathlessness, chest pain or a new cough that does not go away.

Yellowing skin or eyes

Along with dark urine or pain under the right ribs, which can mean liver strain.