There are substances in grapefruit that can interfere with the way your body absorbs and breaks down (metabolizes) certain drugs. This interference allows a higher amount of the drug than usual to circulate in your bloodstream, which can occasionally result in serious or life-threatening adverse reactions.

As little as one glass of grapefruit juice (8 oz. or 250 mL) can cause an increased blood drug level and the effects can last for three days or more. Therefore, even if you drink the juice in the morning and do not take your medication until bedtime, the level of the drug in your blood could still be affected.

The effects vary from one person to another, from one drug to another, and from one grapefruit juice preparation to another. This results in an unpredictable increase in blood drug level, which in some cases can cause serious effects.

The safe course, therefore, is not to take certain drugs with grapefruit juice. Although interactions between grapefruit juice and certain drugs are well documented in the medical and scientific literature, this information may be unknown to the public.

There are several substances in grapefruit which interfere with the way your body handles certain drugs. Consuming grapefruit juice (fresh or frozen) or grapefruit sections can increase, or less commonly decrease, the effects of some drugs, which could lead to serious or even life-threatening adverse reactions.

Certain drugs and health products used in the treatment of the following medical conditions are known to cause this effect:

• Vinblastine (for combating cancer)
• Cyclosporine (for supressing organ rejection following transplant)
• Losartan (for controlling high blood pressure)
• Digoxin (for treating congestive heart failure)
• Fexofenadine (for alleviating allergy symptoms)

The best course to follow is NOT to drink grapefruit juice or eat grapefruit in any form, if you are taking medication for any of the conditions listed above, until you have talked to your doctor or your pharmacist about the potential for an adverse reaction.

Also as a precaution, AVOID taking any drug with grapefruit juice until you have asked your doctor or pharmacist if it is safe to do so.

While sweet oranges and their juice do not appear to cause the same reaction, sour orange juice such as that from Seville oranges, may have an effect similar to grapefruit juice. Tangelos are a hybrid of grapefruit and may also interfere with drugs. Most other citrus fruits, such as lemons, limes, citrons, naturally sweet oranges and tangerines are considered safe.

What To Do

Take the following precautions to minimize the risk of adverse effects when consuming grapefruit or its juice:

• If you are taking medication for any of the conditions listed above, DO NOT drink grapefruit juice or eat grapefruit in any form until you have talked to your doctor and your pharmacist about the potential for an adverse reaction.

• Avoid taking any drug with grapefruit juice until you have talked to your doctor or pharmacist.

• Read the labels on foods and natural health products to make sure they do not contain grapefruit or its juice, or Seville oranges.

• Always report any adverse drug reaction to your doctor or pharmacist immediately.

If you drink grapefruit juice regularly, it would be a good idea to inquire about its possible interaction with any medications you may be taking or any new drugs which are added. Some sources recommend not ingestion grapefruit juice within 2 hours before and 5 hours after a drug which may interact with it. A safer approach would be to substitute another citrus juice, such as orange juice, which has the same vitamins but has not demonstrated the drug interactions. Remember that eating grapefruit or taking grapefruit supplements may also interact with the same medications since the flavonoids could be present in these, as well. You should also be aware that some drinks which are flavored with fruit juice could be flavored with grapefruit juice even if grapefruit is not part of the name of the drink. Check the label for the ingredients, if you are not sure.