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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 and some drinks 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 of medicine in your blood to potentially dangerous levels, or levels that are too low to be effective. 

Some foods can neutralise the effect of certain medicines. Some examples of foods or drinks which can interact with medicines include:

  • grapefruit juice
  • cranberry juice
  • foods high in vitamin K, such as leafy green vegetables
  • salt substitutes or food supplements high in potassium, such as bananas

You should follow the instructions you receive with your medicine. If you're unsure how to take your specific medicine, or whether you can have anything to eat or drink before taking it, speak to your GP or pharmacist (chemist), or phone NHS 111 for advice.

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.

Further information:

Page last reviewed: 30/04/2015

Next review due: 29/04/2017