You’ve been told you have IBS. Or you suspect you do. But the symptoms — bloating, diarrhea, cramping — aren’t unique to IBS. They overlap heavily with IBD (inflammatory bowel disease) and SIBO (small intestinal bacterial overgrowth). The treatment plans are completely different. Misdiagnosis costs months and sometimes years.
This guide gives you the practical differences — what each condition feels like, how it’s diagnosed, and red flags that tell you it’s not IBS.
Quick comparison
| IBS | IBD | SIBO | |
|---|---|---|---|
| What it is | Functional disorder (no visible damage) | Autoimmune inflammation of bowel wall | Bacteria growing where they shouldn’t (small intestine) |
| Diagnosis | Symptoms match Rome IV criteria + rule out others | Endoscopy + biopsy showing inflammation | Hydrogen/methane breath test |
| Pain pattern | Relieved by bowel movement | Often persistent, worse at night | Worse 1-2 hours after meals |
| Blood in stool | Almost never | Common (especially UC) | No |
| Weight loss | No | Yes (often significant) | Mild to moderate |
| Inflammation markers (CRP, calprotectin) | Normal | Elevated | Usually normal |
| First-line treatment | Low-FODMAP, peppermint oil, antispasmodics | Anti-inflammatory drugs, immunomodulators | Antibiotics (rifaximin) |
| Permanent damage risk | None | Yes — can lead to surgery | None directly, but causes nutrient malabsorption |
IBS — the most common, least dangerous
About 10-15% of adults have IBS. It’s a functional disorder — meaning your bowel works incorrectly but doesn’t show inflammation or damage on tests.
Hallmarks:
- Symptoms ≥ 6 months
- Pain related to defecation (better or worse after pooping)
- Pain accompanied by stool changes (frequency or form)
- Tests are normal: blood work, colonoscopy, calprotectin, CT all clean
- Doesn’t wake you up at night — pain stops when you sleep
Why this matters: IBS doesn’t cause permanent damage, doesn’t shorten your life, and responds extremely well to dietary intervention (70% improvement on low-FODMAP). The trade-off is that there’s no quick “cure” — it’s a long-term management protocol.
IBD — Crohn’s disease and ulcerative colitis
About 1 in 250 adults has IBD. The two main forms are Crohn’s disease (can affect any part of the GI tract, often including the small intestine and ileum) and ulcerative colitis (limited to the colon and rectum).
Hallmarks that scream IBD, not IBS:
- Visible blood in stool (more than just streaks from hemorrhoids)
- Unintentional weight loss of more than 5% in a few months
- Persistent diarrhea > 6 weeks without a clear food trigger
- Night-time symptoms that wake you from sleep
- Fever, fatigue beyond what bloating explains
- Mouth ulcers, joint pain, skin rashes (extra-intestinal manifestations)
- Fecal calprotectin elevated (a stool test you can ask for; a number > 250 µg/g strongly suggests IBD)
- Family history of Crohn’s or UC
The killer point: if you have any red flag, you need a colonoscopy. Don’t manage IBD with low-FODMAP — you’ll get nowhere and the disease will progress untreated. IBD treated late causes structural damage to the bowel that surgery may not fully repair.
SIBO — the great impersonator
SIBO is bacterial overgrowth in the small intestine (where bacteria are normally sparse). It causes symptoms nearly identical to IBS-D, which is why 30-50% of “IBS-D” patients actually have SIBO that wasn’t tested for.
Hallmarks that point to SIBO:
- Heavy bloating that comes on within 1-2 hours of eating (faster than typical IBS)
- Significant belching, especially of foul-smelling gas
- Diarrhea alternating with constipation (methane-dominant SIBO)
- Iron and B12 deficiency despite a normal diet (the bacteria steal nutrients before you absorb them)
- Rosacea, fatigue, brain fog — sometimes seen with SIBO
- Got significantly worse after a course of antibiotics that killed your good bacteria
- Got significantly worse after a stomach bug
How to test: Lactulose breath test or glucose breath test. You drink a sugar solution and breathe into a bag every 15-20 minutes for 3 hours. Hydrogen and methane levels indicate which bacterial profile is overgrown.
Treatment: Rifaximin (550 mg × 3 times daily for 14 days) is the gold standard. It’s a non-absorbable antibiotic — works locally in the gut, doesn’t disrupt your whole-body microbiome. Methane-dominant SIBO often needs rifaximin plus neomycin or metronidazole.
Catch: SIBO recurs in 40-60% of patients within a year. The root cause needs to be addressed (slow gut motility, anatomical issues, low stomach acid, etc.).
The diagnostic order
If you have GI symptoms and you don’t know what’s going on, here’s the order to investigate:
Step 1: Rule out red flags (IBD or worse)
Get these baseline tests from your GP or GI:
- Complete blood count (CBC) — looks for anemia, infection
- CRP and ESR — general inflammation markers
- Fecal calprotectin — bowel-specific inflammation; under 50 µg/g normal, over 250 suggests IBD
- TSH — rule out thyroid disease (causes diarrhea or constipation)
- Celiac antibody panel (tTG-IgA + total IgA) — rule out celiac disease before going low-FODMAP (low-FODMAP makes celiac antibodies un-detectable)
- Stool ova and parasites — rule out giardia or other infections
If any of these are abnormal → colonoscopy / endoscopy / further workup. Don’t move to step 2 yet.
Step 2: If everything is normal → consider IBS
Apply Rome IV criteria. If your symptoms fit (and they’re chronic, > 6 months), start low-FODMAP. Give it 4 weeks strict.
Step 3: If low-FODMAP doesn’t help → SIBO breath test
About one-third of “IBS” patients get little to no benefit from low-FODMAP. The most common reason is undiagnosed SIBO. Get the breath test. If positive, treat with rifaximin and see if symptoms resolve.
Step 4: Still nothing? Less common possibilities
- Bile acid malabsorption (BAM) — causes urgent diarrhea after meals, especially mornings. Tested by SeHCAT scan or trial of cholestyramine. Up to 30% of IBS-D is actually BAM.
- Microscopic colitis — common in women over 50; colonoscopy looks normal but biopsies show lymphocytic or collagenous inflammation. Symptoms: chronic watery diarrhea.
- Pancreatic exocrine insufficiency — fatty stools, weight loss; tested by fecal elastase.
- Pelvic floor dysfunction — especially in chronic constipation; needs anorectal manometry.
What to bring to your appointment
GI doctors are time-pressed. Show up with:
- A symptom log of 2-4 weeks — every meal, symptom severity 0-10, stool form (Bristol scale), bowel movement timing
- A list of every medication and supplement you take (including probiotics)
- Family history — IBD, celiac, colon cancer in first-degree relatives
- Specific questions: “Should I get fecal calprotectin?” “Have we ruled out SIBO?” “Is celiac on the table?”
This drastically changes the quality of your visit. A doctor with raw data in front of them runs better differentials than a doctor with vague symptom descriptions.
EaseGut’s symptom log generates this kind of report automatically — at the bottom of every weekly summary you can export a PDF for your appointment.
Bottom line
- IBS is the most common, least dangerous, and most responsive to diet.
- IBD has structural damage and red flags — don’t miss it. Calprotectin + colonoscopy are key.
- SIBO mimics IBS but needs antibiotics. Breath test before assuming low-FODMAP failure.
If you’re not sure which one is yours, the workup matters more than the label. A 30-minute visit with a GI armed with a symptom log usually gets you to the right answer in one round of tests.
The worst path is calling everything “IBS,” self-treating with random probiotics or fiber for years, and never doing the rule-outs. Catch IBD early. Test for SIBO when low-FODMAP fails. Move on from “IBS” if treatment doesn’t track.
References: Lacy BE et al. Bowel disorders. Gastroenterology. 2016;150:1393-1407. Pimentel M et al. ACG clinical guideline: small intestinal bacterial overgrowth. Am J Gastroenterol. 2020;115:165-178.