Sulphanilamide is the alternative British spelling for sulfanilamide, the parent compound of the sulfonamide antibiotics used to treat bacterial and protozoal infections in animals. The sulph- form appears in older British pharmacopoeias and veterinary literature, while sulf- is the modern international spelling. Both refer to the same folic-acid-blocking antimicrobial, often used in combination with other sulfonamides. Awareness of the spelling pair matters when searching historical residue data or interpreting vintage product labels. Modern sulfonamide therapy builds directly on the pharmacological principles this pioneering compound first established.