Medical Oncologist
Gut and bowel
Alternating diarrhoea and constipation
Two different forces act on the bowel at different points in the cycle. The chemotherapy irritates the gut lining and causes loose motions; the anti-sickness medicines slow the bowel and cause constipation. They peak at different times, so the bowel swings. Treating each swing as it arrives makes them bigger — what works is steadying it from underneath.
NG
Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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The short answer
Why does the bowel swing between both extremes?
Because two different forces are acting on it at different points in the cycle. The chemotherapy irritates the gut lining and causes loose motions. The anti-sickness medicines given alongside it slow the bowel and cause constipation. They peak at different times, so the bowel swings between them.
Painkillers, eating very little, drinking too little and lying down all day add to the constipation side. The result is a week that starts with loose motions and ends with nothing moving at all, and it is confusing enough that people often treat the wrong one.
The trap to avoid
Treating each swing as it arrives — an anti-diarrhoeal on the loose days, a laxative on the blocked ones — makes the swings bigger. The bowel is pushed hard in each direction in turn and never settles. What works better is steadying it from underneath.
Keep a simple daily note of what happened. Over a cycle it shows a pattern that can actually be planned around.
!
One pattern that must not be treated at home
Small amounts of liquid motion leaking past while the person still feels blocked, with a swollen or hard stomach, can mean a blockage rather than diarrhoea. Do not give an anti-diarrhoeal. Go to the nearest emergency department the same day and say clearly that they are on chemotherapy. The same applies to any fever, blood in the motions, severe stomach pain, or vomiting where nothing stays down.
Not sure whether this applies to you?
Ask an oncologistWorking it out
Which one do you actually treat?
Ask your team to decide the overall approach rather than reacting to each day. In most cases they will aim to keep the bowel gently regular rather than chasing each swing, which usually means treating the constipation steadily and leaving the loose days to settle on their own with fluids.
Why constipation is usually the one to manage
Because it has an ongoing cause — the anti-sickness medicine — that is present throughout, while the loose motions are a shorter reaction to each cycle. Steadying the constipation side often reduces both, because a bowel that is loaded produces exactly the overflow that looks like diarrhoea.
When the loose motions need treating instead
If they are frequent, not settling, or causing dehydration, they need attention in their own right. But not with a medicine from the drawer, and never when there is a fever. That combination needs assessment the same day.
The question to ask
"My bowels are swinging between loose and blocked through the cycle. What do you want me to do on each kind of day?" Ask it once, write the answer down, and stop making the decision fresh each morning.
Why a written plan matters more here than elsewhere
With most side effects the right response is the same every time. With this one it changes depending on which phase you are in, and the wrong choice actively makes things worse. Having the answer already written on a sheet, agreed with your team, is what stops the household improvising at eleven at night with whatever is in the medicine drawer.
Nothing on this page should be used to start, stop or change a medicine you have been prescribed.Steadying it
What helps whichever way the bowel is going
These are the measures that do not have to be reversed when the pattern flips, which is what makes them useful here.
Fluid, steadily
The one thing that helps both. Loose motions take fluid out; constipation gets worse without it. Sip through the day rather than drinking at meals, and use rehydration solution on the loose days if your team agrees.
Judge it by
- Urine pale and passed regularly
Plain, regular meals
Small amounts at roughly the same times each day settles a bowel better than large irregular meals. Kanji, curd rice, khichdi and soft dal suit both phases without needing to be swapped.
Gentle walking
Helps a slow bowel and does not worsen a loose one. Short walks several times a day, inside the house if that is all that is possible, do more than one long effort.
Check with your team about activity during the low-count days.Protecting the skin
Both extremes make the area around the back passage sore. Wash with water rather than paper, pat dry, and report any break in the skin, because it is an infection route during low counts.
A review of the medicine list
Painkillers, iron and anti-sickness medicines all push towards constipation. Take the whole list to your oncologist — a timing change or a swap often reduces the swing more than adding anything.
Writing it down
Date, what happened, and anything taken. A cycle's worth of that turns a confusing week into a pattern your team can plan around, and stops you guessing each morning.
Being straight with you
What this page cannot tell you
It cannot tell you what to take on which day. That depends on your treatment, your blood counts, what is causing each swing and everything else you are on, and getting it wrong in this particular situation is easier than in most. Ask your team for a plan rather than assembling one from a page.
It also cannot tell you whether the swing is the treatment or something else. An infection, a partial blockage, a medicine you were given for an unrelated problem and conditions unconnected to the cancer can all produce this pattern. Working that out needs blood tests and sometimes a scan.
What to do next
Start the written record today — date, what happened, what was taken. Get the fluid steady and the walking going, since neither has to be reversed when the pattern flips. Take the record and your full medicine list to the next appointment and ask for a single plan covering both kinds of day.
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Questions we are asked
Common questions about a swinging bowel
Is it normal to swing between the two?
It is common and rarely explained to anyone in advance. Two forces are acting at different points in the cycle: the drugs irritating the gut lining, and the anti-sickness medicines slowing it down. They peak at different times, so the bowel moves between the extremes across a single cycle.
Should I take a laxative and an anti-diarrhoeal as needed?
Not as a self-managed routine. Pushing the bowel hard in each direction in turn makes the swings larger and it never settles. Ask your team for one plan covering both kinds of day, and never take an anti-diarrhoeal when there is a fever.
Loose motions after days of constipation — which is it?
Be careful with this one. Liquid can leak past a hard blockage, so loose motions following constipation may be a sign of it rather than the opposite. Do not give an anti-diarrhoeal. If the stomach is swollen or hard, or wind has stopped, go in the same day.
Can I predict which days will be which?
Often, after a cycle or two of writing it down. Most people find the pattern repeats closely enough to plan around — knowing which days to prepare soft food for, when to push fluids, and when to expect the slow phase. That is the main practical use of the record.
Does diet need to change every few days?
Less than people think. Plain regular meals suit both phases — kanji, curd rice, khichdi, soft dal. Add the soaked raisins and extra fibre on the slow days and drop them on the loose ones, but the basic food does not need reinventing twice a week.
Could this be something other than the treatment?
Possibly, which is why a pattern that is new, worsening or not fitting the cycle should be reported. Infections, a partial blockage and conditions unrelated to the cancer can all produce it. Blood tests and sometimes a scan are what sort it out.
Will it settle after treatment ends?
For most people yes, as the anti-sickness medicines stop and the gut lining repairs. It usually eases over weeks rather than months. Ongoing bowel changes well after treatment has finished should be reported rather than accepted as the new normal.
What should we record?
The date, whether motions were loose, normal or absent, roughly how many, anything taken for it, temperature if anyone felt unwell, and how much was drunk. One line a day. It is the single most useful thing to bring to an appointment about this.
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Sources
- National Cancer Institute — Gastrointestinal Complications (PDQ)
- Cancer Research UK — Bowel changes and cancer drugs
- Macmillan Cancer Support — Bowel changes during cancer treatment
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