Most medicines work best when they are kept cool, dry and away from direct sunlight. The bathroom cabinet is actually one of the worst places, because the heat and humidity from showers can break medicines down faster.
Keep tablets and capsules in their original packaging until you take them — the strip and label protect them and remind you of the expiry date. Store syrups and drops as written on the label; some need refrigeration after opening, while others must never be frozen.
Always keep medicines out of the reach of children, ideally in a locked or high cabinet. Check expiry dates every few months and safely discard anything that is out of date or has changed colour, smell or texture. When in doubt, ask your pharmacist.