Diarrhoea on abemaciclib: the side effect that defines it

Diarrhoea, or loose motions, is the side effect most closely linked with abemaciclib. In the large trials, most people taking it had some diarrhoea, though for the majority it was mild or moderate, and only a small share had severe episodes. It usually starts early, often within the first week, and it tends to be most troublesome in the first month. After that, many people find it settles into something occasional and manageable. The reason lies partly in how the medicine acts on the fast-renewing cells lining the gut. The good news is that it responds well to a simple, well-established plan. Before you start, your team will usually advise you to keep an anti-diarrhoea medicine such as loperamide at home. At the very first loose stool, you start it as instructed, drink extra fluids and let your team know. If the loose motions do not improve within about a day, or are severe, your team may pause abemaciclib until things settle and then restart it, sometimes at a lower dose. Dose reductions for diarrhoea are common and do not appear to take away the benefit of treatment. Very few people need to stop abemaciclib because of diarrhoea alone when it is managed promptly.

It starts early

The first week or two is when loose motions are most likely to begin, so be prepared before your first dose.

Act on the first loose stool

Waiting to see if it passes is the most common mistake. Early treatment prevents dehydration.

It usually gets easier

For most people, loose motions become less frequent after the first month or two.

This is a side effect that most people will meet. Planning for it is more useful than hoping to avoid it.

It can affect daily life

Early on, urgency can make travel, work and social events harder. Many people plan around toilet access for a few weeks.

Diet alone rarely fixes it

Food changes help, but medicine and sometimes dose changes do most of the work.

It is not always the medicine

Infections can also cause loose motions, especially with a fever.

What this page cannot tell you

It cannot set your anti-diarrhoea dose or decide when to pause treatment. Your team decides that.

Contact your team straight away if you notice any of these alongside diarrhoea.

Signs of dehydration

Dizziness when standing, very dark or very little urine, a dry mouth or confusion.

Fever or shivering

This could mean an infection while your white cell count is low.

Blood or severe pain

Blood in the stool, black stools or strong tummy cramps need prompt assessment.