Quick answer: Bowels that swing between constipation and diarrhea are most often irritable bowel syndrome with mixed bowel habits (IBS-M). But a hard, stuck stool can also cause "overflow" diarrhea, where liquid stool leaks around the blockage and looks like ordinary diarrhea. Medicines, diet changes and stress can also tip you either way. Because the cause changes the treatment, a lasting change in bowel habit should be checked rather than self-treated.
One week you can't go. The next you can't stop. Or you strain for days, then have a sudden, urgent, watery bowel movement that feels like a reward and a punishment at once.
Swinging from one extreme to the other is common, and it's confusing, because the usual advice for each end of the spectrum can make the other worse. Fiber helps one and aggravates the other, and an anti-diarrheal can be the wrong move entirely.
Key takeaways
- IBS-M (mixed) is defined by having both firm or lumpy stools and runny stools, each more than a quarter of the time, on days with abnormal bowel movements [1].
- Diarrhea right after days of constipation can be "overflow": liquid stool leaking around a hard mass. Harvard Health notes this can be mistaken for diarrhea [3].
- If your diarrhea might be overflow, don't reach for an anti-diarrheal without medical advice.
- A change in bowel habit, whether diarrhea, constipation or both, that lasts more than a few days is a recognized symptom to have evaluated, especially with blood, weight loss or fatigue [5].
- A timeline of stool type and no-poop days shows your swing pattern, which is far more useful to a doctor than "it's all over the place."
Why do I go from constipation to diarrhea?
The most common reason is irritable bowel syndrome with mixed bowel habits (IBS-M), but medicines, diet changes, stress, infections and impaction can all produce the same swing. The pattern alone doesn't tell you which one.
IBS-M is one of three IBS subtypes. NIDDK defines it by stool pattern: on days with at least one abnormal bowel movement, more than a quarter of stools are firm or lumpy and more than a quarter are runny [1]. IBS involves repeated abdominal pain along with changes in bowel movements, and no visible damage in the digestive tract [1]. About 12% of people in the United States have IBS [1]. The NHS notes symptoms can come and go, with periods of flare-ups, and lists alcohol, caffeine, spicy or fatty meals, stress and anxiety, and regular antibiotic use as possible triggers [4]. For the other subtypes, see IBS-C vs IBS-D.
Other things that can swing you from one end to the other:
| Possible cause | How it can produce the swing |
|---|---|
| IBS-M | Mixed stool types, often with pain that eases after going; flares come and go [1][4] |
| Impaction with overflow | Hard stool stuck in the bowel, with liquid stool leaking around it [3] |
| Medicines and supplements | NIDDK lists many that can cause constipation (iron, some antacids, narcotic pain medicines, some antidepressants, calcium channel blockers) [2]; others, such as antibiotics and magnesium antacids, can cause diarrhea [6]. Starting or stopping them can swing your bowels |
| Diet changes | NIDDK lists low fiber, low fluid intake and changes in diet among causes of constipation [2]; sugar alcohols and intolerances can cause diarrhea [6] |
| Routine disruption | Travel, ignoring the urge to go, pregnancy and aging are listed as constipation factors [2] |
| An infection or gut condition | Celiac disease, inflammatory bowel disease, bacterial overgrowth and others can cause diarrhea or constipation [2][6] |
| A colon narrowing or tumor | Rare but important; this is why persistent change needs evaluation [2][5] |
What is overflow diarrhea?
Overflow diarrhea is liquid stool leaking around a hard mass of stool that is stuck in the bowel. Harvard Health describes it as a symptom of fecal impaction: "the stool is leaking around the impacted mass of feces," and notes this can be mistaken for diarrhea [3].
This matters because the usual diarrhea response is the wrong one. If the real problem is a blockage, stopping the flow could make things worse. That's why it's safer to have a doctor sort out which situation you're in, particularly if you've been constipated for days before the loose stool began.
According to Harvard Health, impaction can also cause abdominal pain (especially after meals), a persistent urge to move the bowels, nausea and vomiting, poor appetite, weight loss and feeling unwell [3]. It is more likely in older and bedridden people, and long-term constipation can progress to impaction [3]. NIDDK also lists "a feeling that not all stool has passed" as a constipation symptom [2].
Clues that diarrhea might be overflow:
- Several days of constipation or hard stools came first
- The loose stool is small in amount, frequent, and leaves you feeling not-quite-empty
- A swollen or painful belly, nausea or vomiting
- You are older, less mobile, or on medicines that slow the gut (for example, narcotic pain medicines) [2][3]
None of these proves it. If several fit, contact a doctor rather than treating it yourself. Harvard Health advises calling a doctor immediately if bowel movements stop and abdominal pain or swelling develops, or if there's any rectal bleeding [3].
Is constipation then diarrhea normal?
It's common, but a lasting change in bowel habit shouldn't be written off as "just IBS" without an evaluation. The American Cancer Society lists "a change in bowel habits, such as diarrhea, constipation, or narrowing of the stool, that lasts for more than a few days" among the symptoms of colorectal cancer, alongside rectal bleeding, blood in the stool, weakness and fatigue, and unintended weight loss [5]. The same page notes these symptoms can have other causes, such as infection, hemorrhoids or IBS, so a doctor should evaluate them [5].
This isn't a reason to panic. Most swings turn out to be IBS, medicines or diet. It's a reason to track the pattern and mention it if it persists. The NHS advises seeing a GP if you think you may have IBS and have had symptoms for more than 4 weeks [4].
How to track constipation and diarrhea together
Log every bowel movement with a Bristol type, and log the days with no bowel movement too. Most paper diaries only record the "events," so the constipated stretch is invisible. The swing becomes visible when the empty days, the hard stools and the loose stools sit on one timeline.
What to record each day
- Every bowel movement. Time, Bristol type (1-2 hard, 3-4 typical, 5-7 loose), and urgency or straining.
- No-poop days, marked explicitly.
- Pain and bloating (0-10) and whether going relieved them.
- Medicines and supplements, including laxatives, fiber, iron, magnesium, antacids and anti-diarrheals, with the day you started or stopped.
- Fluids, fiber and big food changes.
- Stress, sleep, travel and, if relevant, your cycle.
If you're unsure how to read the types, our guides to Bristol type 6 poop and how to track bowel movements explain them.
Illustrative 10-day log (example data, not a real person)
| Day | Bowel movements (Bristol type) | Pain/bloat (0-10) | Notes |
|---|---|---|---|
| 1 | None | 4 | Travel day |
| 2 | Type 2, strained | 5 | Little water, few vegetables |
| 3 | None | 6 | Started iron supplement |
| 4 | Type 1, small | 7 | Bloated |
| 5 | None | 7 | Belly tight, nauseous |
| 6 | Type 7 x3, small, urgent | 7 | Felt "not empty" after |
| 7 | Type 6 x2 | 5 | Eased after going |
| 8 | Type 5 | 3 | Back to normal food |
| 9 | Type 4 | 2 | Good day |
| 10 | Type 3 | 2 | Good day |
How to read it: days 1-5 are a constipation run with hard stools and rising pain. Days 6-7 are loose, urgent and small in volume after that run, a shape that is worth raising with a clinician as possible overflow (see above). A new iron supplement on day 3 is another possible contributor [2]. None of this is a diagnosis. Without the log, the story would have been "I had diarrhea last week." With it, a doctor sees the five-day lead-up.
For ongoing records, explore the Get Regular poop tracker. Compare the current logging workflow and access with the details you want to keep. An app record does not identify the cause of a bowel change.
What should I avoid when my bowels swing?
Avoid anti-diarrheals when you might be impacted, and avoid dramatic fiber or laxative changes without a plan. Safe self-care is limited when you don't know which end you're at.
- Don't start an anti-diarrheal after several days of constipation without talking to a doctor, because the loose stool may be overflow rather than true diarrhea.
- Don't stack laxatives to "fix" it. If you use laxatives, note every dose and its timing in your log so a clinician can tell what the laxative did and what your bowel did on its own.
- Change one thing at a time. Fiber, fluids and medicines each move your bowels; changing three at once leaves your log unreadable.
- Keep fluids up, particularly if you have diarrhea; watch for extreme thirst, dark urine, dizziness or urinating less [6].
- Check your medicine list with a pharmacist, since many common medicines can cause constipation [2] and others can cause diarrhea [6].
When to see a doctor
See a doctor if constipation and diarrhea alternate for more than a few days, or at any point if there are warning signs. The American Cancer Society lists a bowel habit change lasting more than a few days among symptoms to have evaluated [5]. The NHS advises a GP visit if you think you have IBS and symptoms have lasted more than 4 weeks [4].
Get care right away for:
- Bowel movements that stop with belly pain or swelling, an inability to pass gas, or vomiting [2][3]
- Blood in the stool, rectal bleeding, or black, tarry stool [2][3][6]
- Severe or constant abdominal pain, or fever [2][6]
- Signs of dehydration, or a change in mental state [6]
- A hard lump or swelling in the belly [4]
Book an appointment soon for:
- A change in bowel habit lasting more than a few days or recurring for weeks [5]
- Unintended weight loss, fatigue or weakness [2][5]
- Diarrhea or constipation that wakes you at night
- A new, lasting change in bowel habit at 45 or older, the age US guidelines start routine colorectal cancer screening [7]
- A family history of colon or rectal cancer, celiac disease or IBD [2]
- Swings that started after a new medicine or supplement
For a fuller checklist, read when to see a doctor about bowel changes. If you haven't gone in days, our guide to not having pooped in days covers that end.
FAQ
Why do I get diarrhea after being constipated?
It can be IBS-M, where stool types alternate [1]. It can also be overflow, where liquid stool leaks around a hard mass of stool [3], or the effect of a medicine, laxative or diet change. The log is the way to tell which one is likely.
Can you be constipated and have diarrhea at the same time?
Yes, in the form of overflow, where liquid stool passes around a hard impaction while you remain constipated [3]. IBS-M can also produce hard and runny stools on the same day, since NIDDK defines it by both stool types occurring on days with abnormal bowel movements [1].
Why do I get explosive diarrhea after constipation?
A backed-up bowel followed by a sudden, large, loose stool is a classic pattern in IBS flares, and overflow can also look like this. Because the cause matters, ask a clinician about it rather than assuming. Note timing, Bristol type, and any pain or bloating in your log.
Is alternating constipation and diarrhea a sign of colon cancer?
Usually not, but a change in bowel habit lasting more than a few days is one of the symptoms the American Cancer Society says should be evaluated, particularly with blood, weakness, fatigue or weight loss [5]. Most causes are benign, such as infection, hemorrhoids or IBS [5].
How do I stop the cycle of constipation and diarrhea?
Start by finding out which pattern you have. A two-week Bristol log, with medicines and no-poop days included, is the best starting point for an appointment. Treatment for IBS-M, overflow and medicine-related swings is different, so it's worth a medical opinion rather than guessing.
Sources
- NIDDK. Definition & Facts for Irritable Bowel Syndrome. https://www.niddk.nih.gov/health-information/digestive-diseases/irritable-bowel-syndrome/definition-facts
- NIDDK. Symptoms & Causes of Constipation. https://www.niddk.nih.gov/health-information/digestive-diseases/constipation/symptoms-causes
- Harvard Health. Constipation and impaction. https://www.health.harvard.edu/digestive-health/constipation-and-impaction-a-to-z
- NHS. Irritable bowel syndrome (IBS): Symptoms. https://www.nhs.uk/conditions/irritable-bowel-syndrome-ibs/symptoms/
- American Cancer Society. Signs and Symptoms of Colorectal Cancer. https://www.cancer.org/cancer/types/colon-rectal-cancer/detection-diagnosis-staging/signs-and-symptoms.html
- NIDDK. Symptoms & Causes of Diarrhea. https://www.niddk.nih.gov/health-information/digestive-diseases/diarrhea/symptoms-causes
- US Preventive Services Task Force. Screening for Colorectal Cancer: US Preventive Services Task Force Recommendation Statement. JAMA, 2021. https://jamanetwork.com/journals/jama/fullarticle/2779985
This article is for general education and isn't medical advice. Get Regular is a tracking app — it doesn't diagnose or treat any condition. If you have red-flag symptoms or ongoing changes in your digestion, talk to a doctor.