Why must some medicines be taken on an empty stomach?

If you have any concerns about the information below, or need any help understanding it and relating it to your own situation, you should talk to your doctor or pharmacist.

The instructions on some medicines state that they must be taken 'before food' or 'on an empty stomach'. Most of the medicines that must be taken on an empty stomach do not pass into the bloodstream very well if there is food in the stomach. Some common examples include:

• Flucloxacillin.
• Penicillamine.
• Levothyroxine (thyroxine).
• Penicillin V (phenoxymethylpenicillin).
• Oxytetracycline.
• Rifampicin.

These medicines should be taken at least 30 minutes (an hour or more is even better) before food. This will allow them to get into the bloodstream before any food is eaten. If you do not do this then the medicine will not be as effective, so you should follow the instructions carefully.

For some medicines, even the smallest amount of food in the stomach greatly reduces the amount of medicine that passes into the bloodstream. A group of medicines called bisphosphonates, used to treat conditions such as osteoporosis, are a particular problem. They include alendronate, clodronate, etidronate and risedronate. These medicines should ideally be taken first thing in the morning before breakfast, as follows:

• Alendronate - Take at least 30 minutes before the first food, drink (other than water), or medication of the day with plain water only.
  
• Clodronate - Take with a little fluid, but not milk, at least 1 hour before or 1 hour after food.
  
• Etidronate - Take with water at least 2 hours after you last ate, but 2 hours before your next meal is due.
  
• Risedronate -Take at least 30 minutes before the first food or drink (other than water) of the day or at least 2 hours after you last ate or drank (other than water) but 2 hours before your next meal is due.

Some medicines, which work in the gut, will not work if they are taken after food.
For example:

• Sucralfate 'coats' stomach and duodenal ulcers to heal them. You must take this at least an hour before eating or it coats the food instead of the ulcer and the treatment does not work.
• Mebeverine is used to reduce bowel spasm at meal times. You should take it 20 minutes before meals, so that it can start working before food is eaten.
• Sodium cromoglicate capsules should be taken at least 30 minutes before meals to reduce the effects of certain types of food allergy.

Summary

• Follow carefully the instructions you receive with your medicines.
  
• For medicines that must be taken on an empty stomach so that they pass into the bloodstream properly, you should choose times of the day that are convenient to you. Before breakfast is ideal for once a day medicines, but other good times include mid-morning, mid-afternoon and last thing at night. Bisphosphonate medicines (see above) should not be taken at bedtime.
  
• Some medicines need to be taken just before meals or they will not work properly.