Can plain couscous give my cat diarrhoea or an upset stomach?

Yes, it can, especially if your cat isn't used to grains. Plain cooked couscous is safe nutritionally, but a sudden new food or a large portion may lead to loose stools, gas, or vomiting due to simple intolerance. Some cats might also have a rare grain allergy, which could cause itchy skin or digestive upset. Offer just a tiny amount first to see how your cat reacts, and if diarrhoea or vomiting occurs, skip it next time and call your own vet if symptoms persist.