Going back and forth between constipation and diarrhea is one of the most dreaded bathroom experiences. You might feel like just as you get one under control, the other takes its place.
IBS-M is one possibility, but diet, medications, stress, and other conditions can cause similar changes. Knowing what to look for can help you decide what to try and when to see a doctor.
Why Does Stool Alternate Between Hard and Loose?
Stool consistency depends partly on how quickly waste moves through your intestines. Slow movement allows more water to be absorbed, making stool hard. Faster movement leaves more water in the stool, making it loose.
A study found that stool form reflects intestinal transit time more closely than bowel movement frequency.
IBS involves changes in communication between the gut and brain, intestinal movement, and sensitivity to digestive activity. These changes in gut function can contribute to abdominal pain and changing bowel habits.
Common Reasons Your Bowel Habits Change
Short-lived changes usually trace back to something that disrupted your routine, your diet, or your hormones.
Travel and routine changes. Different meal times, less fluid, reduced activity, and holding in a bowel movement can cause constipation. Unfamiliar foods may loosen stools.
Diet changes. A sudden increase in fiber, sugar alcohols in sugar-free products, or a large, fatty meal can change stool consistency.
Stress. Stress can affect gut movement and sensitivity, contributing to constipation or diarrhea.
Your menstrual cycle. Hormonal changes can affect bowel habits. In a survey of healthy women, 24% reported diarrhea before menstruation and 28% during it.
A stomach infection. Diarrhea may be followed by temporary changes as you recover. Symptoms that persist or worsen need medical assessment.
Medications and Overflow Diarrhea
What you take can change stool form as much as what you eat, which makes medications worth reviewing early.
Laxatives and anti-diarrheals. Using one to counteract the other can complicate your bowel pattern. Ask your clinician for a plan if you keep switching between constipation and diarrhea.
Other medications. Opioid pain relievers commonly cause constipation. Antibiotics can cause diarrhea, which is sometimes serious enough to need medical attention, especially if it develops during or after a course of treatment.
Overflow diarrhea. Severe constipation can cause fecal impaction, when hardened stool becomes stuck in the rectum. Liquid stool may leak around the blockage and look like diarrhea. This is more common in older adults and people in care facilities, and suspected impaction needs medical assessment, not simply anti-diarrheal medication.
When Could It Be IBS-M?
IBS-M is a subtype of irritable bowel syndrome involving recurrent abdominal pain and both hard and loose stools.
Under the Rome IV criteria, IBS involves abdominal pain, on average, at least one day a week over the previous three months. The pain relates to bowel movements or changes in stool frequency or form, and symptoms must have started at least six months before diagnosis.
For IBS-M, more than 25% of abnormal bowel movements must be hard or lumpy (Bristol stool types 1 and 2), and more than 25% must be loose or watery (types 6 and 7).
Alternating constipation and diarrhea alone does not establish IBS-M. A clinician considers your symptoms and history and decides whether testing is needed.
The 2021 ACG guideline supports diagnosing IBS using symptom criteria and targeted tests when appropriate, rather than automatically ordering extensive tests to rule out every condition.
How IBS Subtypes Shift Over Time
Bowel habits can fluctuate. In a year-long study of women with IBS using older Rome II criteria, more than 75% changed subtype at least once, and half of those initially classified as IBS-M changed subtype within 12 weeks.
Those findings come from one specific group and may not predict what happens to every person with IBS-M.
The ACG recommends keeping a daily stool diary for at least two weeks to help classify IBS subtype. Ideally, classification is based on stool consistency when treatments affecting bowel habits are not influencing the pattern. Don't stop prescribed medication without discussing it with your clinician.
Other Conditions a Doctor May Check For
Which tests a doctor orders depends on your symptoms, your medical history, and your personal risk factors.
Celiac disease. Blood testing may be recommended when IBS symptoms include diarrhea.
Inflammatory bowel disease. For suspected IBS with diarrhea and no warning signs, a clinician may order stool tests such as fecal calprotectin or lactoferrin, sometimes alongside a C-reactive protein blood test, to assess whether further investigation is needed.
Colorectal cancer. Average-risk adults should begin routine screening at 45 under US recommendations. Screening is separate from evaluating symptoms: concerning bowel changes may need assessment even if you're younger or up to date on screening.
