Read the drug chart before you scope the chronic diarrhoea

Chronic diarrhoea generates a standard workup — stool studies, coeliac serology, calprotectin, colonoscopy with biopsies — and a substantial fraction of it is caused by something already on the prescription list.

The common culprits are worth naming because they are easy to skip over: metformin, which causes diarrhoea in a sizeable minority and is often dismissed because the patient has taken it for years; magnesium-containing antacids and supplements; proton pump inhibitors, which cause microscopic colitis and predispose to small bowel bacterial overgrowth; and olmesartan, which produces a sprue-like enteropathy with villous atrophy that looks exactly like coeliac disease on biopsy and resolves only when the drug stops. Add laxatives, colchicine, selective serotonin reuptake inhibitors, and any recent antibiotic course.

The olmesartan case is the one worth holding, because it reaches the end of the workup: a patient with villous atrophy and negative coeliac serology who has not improved on a gluten-free diet should have their antihypertensive reviewed before anything else is considered.

• Take a full drug history including over-the-counter antacids and supplements before investigating
• Ask about duration — a drug taken for years is not thereby excluded
• In villous atrophy with negative coeliac serology, check specifically for olmesartan
• Consider microscopic colitis in anyone on a proton pump inhibitor, and biopsy a normal-looking colon
• Trial withdrawal of the suspected agent before extending the workup, where it is safe to do so

Why it matters

A drug cause is the one diagnosis the full workup can miss entirely while returning abnormal results that look like something else.

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