British spelling of 'diarrheal'; relating to or causing diarrhea.
From the British spelling of 'diarrhoea' (Greek 'diarrhoia') + suffix '-al', maintaining the '-oe-' diphthong from older English conventions.
British English keeps the Greek 'oe' spelling in 'diarrhoea' while American English simplified it to 'diarrhea'—this split happened in the 1800s when spelling standards diverged.
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