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Written by Pietro Mignano, pharmacist and Human Nutrition Science student. Intern at Duke Diet and Fitness Center.


In this article, I will try to explain why you should avoid drinking grapefruit juice if you have allergies.

We are talking about food-drug interactions.

Of course, at this point, you want to know: ‘’what is a food-drug interaction?’’

It happens when the food you eat, in some way, improves or decreases the effects of a medication.

Fortunately, this mechanism doesn’t happen with all medications but the first thing you should know is that it can happen with prescriptions, over-the counter drugs and supplements too!


This means that it is very important to communicate to your dietitian if you are taking any medications, drugs or supplements.

The majority of drugs are metabolized in and eliminated from the liver (and the kidney too through urine). In the liver there are some components that are involved in the metabolism of drugs and in the detoxification of the body. The main function of these important components is to modify medications, through complex chemical reactions, in order to be expelled easily from our body.

But what happens when we consume any food that can be linked with the same components?

To date, that other interaction can happen when food can link to the same effect site used by the drug.

Here are some interactions that you should be aware of:



Despite the fact that this fruit has a lot of healthy properties it must be avoided while taking an antihistamine as it contains a few molecules (bergamottin and naringin) that can interfere with the antihistamine metabolism.

This means that if you take your antihistamine pill while drinking a glass of grapefruit juice, the antihistamine may not have effects on you - or, even worse, the antihistamine could work too well and this means you may experience more side effects like drowsiness (very dangerous if you drive a car)!


Ok, this food-drug interaction is probably better known but let’s see how it works. (Just to mention for the rock’n’roll fans, the Rolling Stones song “mother little helper”  referred to this drug).

Commonly known as minor tranquilizers, these medications can help to reduce anxiety and promote a hypnotic effect. To have these effects the benzodiazepine must reach the brain that is the “anxiety controller.”

These drugs are mostly safe but…don’t drink alcohol if you are taking benzodiazepine!

The alcohol can rapidly reach the brain too and dramatically increase the benzodiazepine effects on your body and this can cause:

  • Loss of memory and loss of consciousness (the truth serum used by the FBI uses a similar mechanism)
  • Anxiety (commonly known as the paradox effect)
  • Comatose state




Anticoagulant drugs are very important for those who have had a stroke or for those who have a higher risk for stroke. Indeed these drugs make the blood more fluid to avoid the formation of blood clots.

This is possible because it is able to deactivate vitamin K.

Ok, let’s say that in a simpler way! Vitamin K is responsible for activating the normal coagulative process that occurs in everyone. But for people who have to maintain more fluid blood, this is not good!

People taking anticoagulants should not eat too many leafy green vegetables because they are rich in vitamin K and this counteracts the effects of the anticoagulant.





Tetracycline is a family of antibiotics that was common in the 70’s. This antibiotic has the power (and this is a side effect) to chelate (bind) with some minerals such as calcium. This means that tetracycline has a chemical structure that can bind with calcium and if you drink milk or eat cheese while taking this antibiotic, it will bind with the calcium from this food and become inactive. Of course, this prevents the tetracycline from having the desired medical effect plus this will prevent calcium from being used to produce the positive and healthy effects that are so important for your bones and teeth.



Aminofilin is a common bronchodilator. It allows people who suffer from asthma to breathe easier by dilating the lungs. The aminofilin works in the lungs but also in the blood increasing pressure (this is a side effect that happen if you take an overdose of this medication that is usually safe at common dosage).

The chemical structure of the caffeine in the coffee is very similar to the chemical structure of the aminofilin and can drastically amplify the effect of the drug! So if you suffer from asthma, stop drinking coffee or try to drink decaf.


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