Kidney function can suddenly drop when someone becomes unwell, for example when they get an infection or diarrhoea and vomiting. This is particularly likely when the person is elderly, has other medical problems or is taking certain medication. This drop in kidney function is called acute kidney injury (AKI) and is associated with a higher chance of being admitted to hospital and spending longer in hospital. Expert opinion suggests that something as simple as stopping certain medications during illnesses may reduce the chance of AKI (sick day rule). However, there are no studies to prove that this protects patients from AKI and other experts actually warn that stopping these medications may cause harm in a different way. This study will explore whether prescription of these medications increases the likelihood of AKI. This will provide indirect evidence on whether stopping these medications when a patient becomes unwell may reduce the likelihood of AKI.