The recipe for Nin Jiom Pei Pa Koa has not changed since it was first concocted in the 1600s. Yet the sweet Chinese cough syrup, which has the colour and consistency of mud, is enjoying a renewed surge in popularity. Sales rose by almost a third between 2019 and 2024, reaching 635m yuan ($88m) last year. Those coughing up for the syrup are not just in China, but increasingly in the West, too.