Period poop, diarrhoea and bloating: what your cycle does to your gut

Q: What helps with period diarrhoea and constipation?

Treat the cause where you can, and keep it simple: Period pain relief: ACOG says NSAIDs such as ibuprofen and naproxen lower the body's prostaglandins and work best taken at the first sign of your period or pain. People with asthma, stomach ulcers, bleeding disorders, aspirin allergy, or liver damage should not take NSAIDs, so check with a pharmacist first. Fluids: the NHS advises drinking plenty of fluids with diarrhoea, and a pharmacist can suggest rehydration sachets if you show signs of dehydration. For constipation: the NHS suggests gradually increasing fibre, drinking enough fluids, staying active, and not delaying when you feel the urge to go. Hormonal options: for people with endometriosis, the NHS and NICE both list hormonal treatments, such as the combined pill, as ways to ease pain. That is a decision to make with a clinician.

Yes, your period can change your poo. Looser stools, more frequent trips to the toilet, and cramping that seems to sit in your bowel as much as your womb are common in the days around a period, and many people notice the opposite, a slower, more constipated, more bloated gut, in the week or so before bleeding starts. The best-supported explanation for period diarrhoea is prostaglandins, the same chemicals that cause period cramps.

The pre-period constipation side is less settled. Rising progesterone after ovulation is the usual explanation, and one small study did find slower gut transit in the luteal phase, but other studies found little or no change. A pattern that comes and goes with your cycle is usually not a sign that anything is wrong. Two things are different: pain when you poo that follows your cycle, and any bleeding from your bottom. Take those to a doctor rather than putting them down to your period.

A person resting both hands on their lower abdomen. Photo by Polina Zimmerman on Pexels.

How common are bowel changes around a period?

More common than the silence around them suggests. A 2014 study in BMC Women's Health surveyed 156 women attending a gynaecology clinic for routine care, none of whom had a known gut, gynaecological, or psychiatric condition. Asked about the five days before a period and the days of bleeding:

Any gut symptom: 73% had abdominal pain, diarrhoea, constipation, nausea, or vomiting at one or both times.
Abdominal pain: 58% before the period and 55% during it.
Diarrhoea: 24% before the period and 28% during it.
Fatigue: about half at both times, and it was linked to having several gut symptoms at once.

People with low mood or anxiety around their period were more likely to report several gut symptoms. The authors suggest the brain, gut, and hormones may share pathways, which is a hypothesis, not a finding.

It was a single-clinic survey without hormone measurements, so it shows these symptoms are common, not why they happen.

Why does your period give you diarrhoea?

The explanation with the most support is prostaglandins. The American College of Obstetricians and Gynecologists (ACOG) describes them as chemicals made in the lining of the uterus that make its muscle and blood vessels contract. Levels are highest on the first day of a period and fall as the lining sheds, which is why cramps usually ease after the first few days.

The bowel is made of the same kind of smooth muscle, and it sits right next to the uterus. ACOG notes that severe period pain can come with diarrhoea, nausea, and vomiting. The likely reading is that prostaglandins act on the gut as well as the womb. That is an inference, because no study has neatly measured prostaglandins in the bowel against stool frequency. It does fit the way period diarrhoea and bad cramps often arrive and settle together.

Why do you get constipated and bloated before your period?

The usual explanation is progesterone. It rises after ovulation and relaxes smooth muscle, so the theory is that it slows the gut in the second half of the cycle. Then it drops just before a period, and the bowel speeds up again. The NHS lists bloating among the most common PMS symptoms.

The evidence is more mixed than most articles admit:

Slower in the luteal phase: a 1981 study in Gastroenterology of 15 women found food took significantly longer to reach the large bowel when progesterone was high.
No clear difference: a 1989 study in Gut of 18 women with confirmed ovulation found no significant change in whole-gut transit time, stool weight, or bowel frequency between cycle phases.
Small at most: a 1996 study in Gut concluded that any effect of menstrual hormones on transit was small and of doubtful clinical significance next to normal day-to-day variation.
Bigger in IBS: a 2014 review found that ovarian hormones change gut sensation and movement in both healthy people and people with irritable bowel syndrome, but that the mechanisms are still poorly understood.

So progesterone may play a part, but it is probably not the whole story. The NHS lists low fibre, low fluid intake, inactivity, routine changes, and stress among the most common causes of constipation. If you already have IBS, cycle-linked changes may be more noticeable. The NHS notes that IBS causes flare-ups of pain, bloating, diarrhoea, and constipation.

Our guide to bloating before your period covers the fullness and water-retention side in more detail.

Which bowel symptoms point towards endometriosis?

Endometriosis is a condition where tissue similar to the womb lining grows elsewhere, and the NHS explains that it sometimes affects the bowel. It lists pain when you poo or pee among its symptoms. ACOG also lists pain during bowel movements, along with constipation and bloating.

The NICE guideline on endometriosis tells clinicians to suspect endometriosis when someone has "period-related or cyclical gastrointestinal symptoms, in particular, painful bowel movements". Other signs on that list include chronic pelvic pain, period pain that affects daily life, and deep pain during or after sex.

Bowel pain that repeats with your cycle is a reason to ask about endometriosis, even if your symptoms have been put down to IBS. The NHS notes that endometriosis can look like IBS, which is one reason diagnosis often takes a long time. You do not need to prove anything before you ask. The NHS says to see a GP if you think you might have endometriosis or if your symptoms affect your everyday life.

Endometriosis UK lists pain on opening the bowels and, in some cases, rectal bleeding during a period as symptoms of bowel endometriosis. Bleeding from the bottom has several other causes, so do not assume which one it is. The section below explains how quickly to get it checked.

For a wider checklist, see endometriosis symptoms worth tracking.

What is normal and what needs a doctor?

These thresholds come from NHS guidance on bleeding from the bottom, bowel cancer symptoms, diarrhoea, and endometriosis.

What you notice	What it likely means	What to do
Looser, more frequent poo in the first day or two of bleeding, easing as cramps ease	A common pattern linked to prostaglandins	Track it; nothing urgent
Constipation or bloating in the week before your period, easing once bleeding starts	Common and cycle-linked, though the cause is debated	Track it; try fibre, fluids, and movement
Pain when you poo that repeats with your cycle	A recognised sign of possible endometriosis	See a GP and say it follows your cycle
Blood in your poo for 3 weeks, or poo that is softer or thinner than usual for 3 weeks	Not a period symptom	See a GP
Black or dark red poo, or bloody diarrhoea	Not a period symptom	Urgent GP appointment or NHS 111
Diarrhoea lasting more than 7 days, or signs of dehydration that do not improve	Not explained by your cycle	Call NHS 111
Heavy, non-stop bleeding from your bottom, or large clots in the toilet	An emergency	Call 999 or go to A&E

The NHS also lists unexplained weight loss, unusual tiredness, and a lump in your tummy as reasons to see a GP. A cycle pattern does not rule out another cause. If your bowel habit has changed and stays changed through the whole month, that is not a period pattern.

What helps with period diarrhoea and constipation?

Treat the cause where you can, and keep it simple:

Period pain relief: ACOG says NSAIDs such as ibuprofen and naproxen lower the body's prostaglandins and work best taken at the first sign of your period or pain. People with asthma, stomach ulcers, bleeding disorders, aspirin allergy, or liver damage should not take NSAIDs, so check with a pharmacist first.
Fluids: the NHS advises drinking plenty of fluids with diarrhoea, and a pharmacist can suggest rehydration sachets if you show signs of dehydration.
For constipation: the NHS suggests gradually increasing fibre, drinking enough fluids, staying active, and not delaying when you feel the urge to go.
Hormonal options: for people with endometriosis, the NHS and NICE both list hormonal treatments, such as the combined pill, as ways to ease pain. That is a decision to make with a clinician.

What is worth recording?

A clear record helps more than a vague memory when you see a doctor. NICE advises that a pain and symptom diary can help conversations about suspected endometriosis. For each day you notice something, record:

Cycle day, so you can see whether symptoms cluster before or during bleeding
Stool change: constipated, loose, diarrhoea, or painful to pass
Gut feeling: bloated, gassy, or nauseous
Pain: where it was and whether it came with passing a poo
Blood: any blood in your poo or on the paper, written as a note

In Flowy's day log, digestion and stool have their own sections rather than being folded into a general symptoms list. Digestion covers bloated, gassy, heartburn, and nauseous; stool covers normal, constipated, loose stool, diarrhoea, and painful. Any bleeding from your bottom goes in a note. No tracker can tell you why your bowel is doing what it is doing. It can show you whether the pattern follows your cycle, which is what a doctor will ask. Our guide to preparing a symptom diary for an appointment covers how to turn a few cycles of notes into a short summary.

Frequently asked questions

Is it normal to poo more on your period?

Yes. Diarrhoea and more frequent bowel movements are common in the days around a period, and prostaglandins are the most likely reason. It usually settles within the first few days of bleeding.

Why does it hurt to poo on my period?

Some discomfort alongside cramps on a heavy day is common. If it hurts every cycle, NICE lists painful bowel movements linked to periods as a reason to suspect endometriosis, so mention it to a GP.

Can endometriosis make you bleed from your bottom?

Endometriosis UK lists rectal bleeding during a period as an occasional symptom of bowel endometriosis, but bleeding from the bottom has many other causes. Get it checked rather than assuming it is linked to your period.

Does period poop mean I have IBS?

No. Cycle-linked bowel changes are common in people without IBS. The NHS suggests seeing a GP if you think you might have IBS and your symptoms have lasted more than four weeks.

The short version

Bowel changes around a period are common and usually harmless. Prostaglandins explain much of the period diarrhoea. Pre-period constipation and bloating are common too, but the evidence on why is weaker than most sources suggest. Track the timing, treat the pain, and take two things seriously: pain when you poo that follows your cycle, and any blood. If cramps are the bigger problem, our period cramps guide covers relief options and their limits.

This article is for general education only. It is not a diagnosis, individualised medical advice, or a substitute for care from a qualified healthcare professional.