The rapid transition from arable to dairy farming in Ireland following the Great Famine presented many challenges which included the spread of animal diseases. Providing solutions to these problems offered opportunities for entrepreneurs throughout Great Britain. This paper explores the origin and decline of a treatment for red water disease in cattle against the changing political and economic situation in Ireland. It draws heavily on correspondence between the Scottish proprietor of the remedy and a range of correspondents in Ireland. The product prospered with the expansion of Irish dairy farming from the 1860s, but came to an abrupt end largely as a result of the ‘economic war’ between Britain and Ireland in 1938. It also shows that some British entrepreneurs went to considerable lengths to defend their export trade to Ireland, not least the trade in veterinary medicines.