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Why must some medicines be taken on an empty stomach?

Some medicines need to be taken "before food" or "on an empty stomach". This is because food can affect the way these medicines work.

For example, taking some medicines at the same time as eating may prevent your stomach and intestines absorbing the medicine, making it less effective. Alternatively, some foods can interact with your medicine, either by increasing or decreasing the amount in your blood to potentially dangerous levels. These include:

  • grapefruit juice
  • cranberry juice
  • foods high in vitamin K
  • salt substitutes or food supplements high in potassium

As a general rule, medicines that are supposed to be taken on an empty stomach should be taken about an hour before a meal, or two hours after a meal. Forgetting these instructions on rare occasions is unlikely to do any harm, but taking these medicines with food regularly may mean they don't work.

You should follow the instructions you receive with your medication. If you're unsure how to take your specific medication, speak to your GP or pharmacist (chemist), or phone NHS 111 for advice.

Further information:

Page last reviewed: 30/04/2015

Next review due: 29/04/2017