Heat, moisture and light can break medicines down faster, so a bathroom cabinet, which gets warm and humid, is often a poor choice despite being common. A cool, dry cupboard away from sunlight is usually better. Keeping medicines in their original packaging protects them and keeps the instructions and expiry date to hand.
Some medicines have special storage needs. Certain liquids, eye drops, insulin and a few others must be kept in the fridge, and some have a limited shelf life once opened. The label or leaflet will tell you, including any temperature range to stay within.
Above all, keep medicines out of sight and reach of children and pets, ideally in a locked or high cupboard. If a medicine has been stored incorrectly, for example left in a hot car, ask your pharmacist whether it is still safe to use.