Most leftovers go off slowly and tell you so with a bad smell. Cooked rice is different. Raw rice can carry the spores of a bacterium called Bacillus cereus. Spores are tough, dormant forms of bacteria, and they survive normal cooking. If the rice then sits warm for a long time, the spores wake up, multiply and can produce a toxin. Rice with that toxin can look and smell completely normal.
This is why batch-cooked rice is a meal-prep classic that needs a little care. The rules are short, and the tool below lets you test your own habits.
Food Standards Scotland gives the core advice for cooked rice: chill it as quickly as possible, ideally within 1 hour; do not keep it in the fridge for longer than 24 hours before eating; and if you warm it up, make sure it is steaming hot all the way through, and never reheat it more than once. General UK food safety guidance also says the fridge should be at 5°C or below.
The aim is to get the rice out of the warm zone fast. A few habits do most of the work:
Heat the rice until it is steaming hot all the way through, with no cold patches in the middle. Stirring halfway helps in a microwave, and a splash of water stops it drying out. Then eat it. Rice that has been reheated once should not go back into the fridge for another round.
Note that the same bacteria can grow in other starchy leftovers such as cooked pasta, so the same habits are sensible for those too. Most healthy adults who get this kind of food poisoning recover within a day or so, but it is unpleasant, and it is more serious for babies, older people and anyone with a weakened immune system.
Cool fast, fridge within the hour, eat within 24 hours, reheat steaming hot, once.
This guide is general food-safety information, not medical advice. If you are pregnant, older, have a weakened immune system or another health condition, follow your national food-safety advice for higher-risk groups and ask your doctor or dietitian if unsure. If you have vomiting or diarrhoea that does not settle, or you are worried about a baby, contact a doctor or your local health service.