Constipation one week, diarrhea the next, and sometimes both in the same day. If that sounds familiar, you're not imagining it. Alternating bowel habits are common in irritable bowel syndrome, and doctors even have a name for this pattern: IBS with mixed bowel habits, or IBS-M.
What Is IBS-M?
Doctors classify IBS into subtypes based on stool form on days with abnormal bowel movements, using the Bristol Stool Form Scale. Under the current Rome IV criteria, IBS-M means that more than a quarter of your bowel movements are hard (types 1–2) and more than a quarter are loose (types 6–7).1 The other subtypes are IBS-C (mostly constipation), IBS-D (mostly diarrhea) and IBS-U (unclassified). Our IBS-D vs IBS-C guide explains the differences.
Why Do Bowel Habits Swing?
IBS symptoms naturally fluctuate, and many people move between subtypes over time. In a 12-month study, the subtype changed in about a third of people who still met the criteria for IBS, and IBS-M was the least stable subtype.2 In one trial cohort, only one in four people kept the same subtype throughout.3 So "constipated this month, loose last month" is not unusual, and it's worth tracking rather than reacting to each change.
Diet, stress, sleep, hormones and medicines can all nudge bowel habits in one direction. A common trap is overcorrecting: taking a laxative for constipation, then an anti-diarrheal for the loose stools that follow, which can keep the cycle going.
Other Causes to Rule Out
Alternating bowel habits aren't always IBS. A doctor will usually want to rule out other causes first. UK guidelines recommend testing for celiac disease in people with IBS-type symptoms, along with basic blood tests.4 Other possibilities include:
- Constipation with "overflow", loose stool leaking around hard stool
- Celiac disease or inflammatory bowel disease
- Side effects of medicines or supplements
- Thyroid problems
See a doctor promptly if you notice any of these warning signs:
- Blood in your stool or black stools
- Unintended weight loss
- Symptoms that wake you at night
- New bowel changes after about age 50
- A family history of bowel cancer, inflammatory bowel disease or celiac disease
What Helps With IBS-M
- The low FODMAP diet. In a network meta-analysis of randomized trials, it ranked first among dietary approaches for global IBS symptoms, abdominal pain, bloating and bowel habit.5 Start with our beginner's guide.
- Soluble fiber such as psyllium. It can firm up loose stool and soften hard stool. In a randomized primary-care trial, psyllium improved IBS symptoms, while bran did not6, and US guidelines recommend soluble fiber for IBS.7 Introduce it gradually.
- Regular meals and enough water, rather than long gaps followed by large meals.
- Tracking. Two weeks of an IBS food diary with Bristol stool types will show your real pattern, and is exactly what your doctor or dietitian will want to see.
Avoid self-treating swings with alternating laxatives and anti-diarrheals without guidance. Ask your doctor or pharmacist for a plan that fits your pattern.
