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The Grapefruit Juice Effect. Drug Interactions and the Dangers of Grapefruit with Lipitor, Allegra, Proscor, Zoloft...

Updated on January 11, 2009

We all know that the dangers of pharmaceuticals are multiplied when taken in combination with other medications – and we are all savvy enough to read up on possible drug interactions before combining two or more medications to avoid any chance of these adverse reactions.

But avoiding grapefruit juice as well?!?

If you take any number of medications that are metabolized by a certain enzyme in the digestive tract, you should avoid grapefruit juice.

What Are Some Drugs That Are Affected by Grapefruit Juice?

This is not a complete list!!! Do not assume that not seeing a certain drug listed below means that is is safe to be taken with grapefruit!

The ADHD medication adderall's potency can be increased when taken in conjunction with grapefruit juice.

Hismanal (Astemizole) is an antihistamine type drug that although once popular, has been taken off of pharmacy shelves due to an intense reaction with grapefruit! People taking alegra should also avoid grapefruit.

Cough medication

Mamy cough medications contain dextromethorphan (Which is also used by teens to get high!) This drug is affected by grapefruit.

Seizure medications

Tegretol (Carbamazepine)


Estradiol (Estrogen)

Heart Medications

  • Amiodarone
  • Quinidine
  • Coumadin (Blood thinner)


  • Diazepam
  • Halcion

Cholesterol Medications

  • Lipitor
  • Zocor
  • Mevacor

Calcium Channel Blockers

  • Pkendif
  • Cardene
  • Verepamil
  • Nifedipine
  • Nimodipine
  • Nisoldipine

Erectile Dysfunction Meds

  • Viagra
  • Cialis


  • Duragesic
  • Actiq
  • Sufenta
  • Alfenta

Depression Medications 

  • Zoloft
  • Luvox

What Causes the Grapefruit Juice Effect?

Eating grapefruit causes an inhibitory reaction with enzyme CYP 3A4 in the digestive tract. If you eat grapefruit, you will have less CYP 3AR available in the walls of your intestines – and these diminished levels will last for about 24 hours after the grapefruit juice consumption.

This enzyme is critical for the breakdown and metabolization of many medications. If you have lowered levels of the enzyme in the intestines, less of the medication is broken down. Less metabolized medications will have unpredictable effects – often showing increases in potency or side effects.

Breaking down less of a drug in the intestines leaves more of the drug available to cause bodily reactions.

Variable Effects

Interestingly, the grapefruit juice affect will affect people very differently, with some people finding a great effect, and others very little. Researchers believe that people have greatly differing levels of available CYP 3AR in the digestive tract, and it is this varying quantity of enzymes that causes the varying sensitivity to grapefruit juice.

What about Other Citrus Fruits?

Only grapefruit seems to have this effect on medications. Researchers aren’t exactly sure what it is in Grapefruit that causes the enzymatic reaction.

What if you are already taking a medication that interacts with grapefruit juice…and you drink grapefruit juice regularly?!?

You should ask your doctor about the grapefruit juice effect prior to starting a new medication.

If you are already taking a medication that supposedly interacts with grapefruit, and you consume grapefruit juice regularly – you should not stop eating grapefruit juice prior to talking with your doctor! Your doctor will want to talk with you about the effects of the drug in conjunction with the fruit, and may or may not ask you to make changes.


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    • lafamillia profile image

      lafamillia 6 years ago from Soutcentral Europe

      This is really ridiculous if you ask me, as a researcher of opioid and their boosters. You have to take 10 or more LITERS to feel SOME "real" difference. But, hey, yes You are right - but in small quantities it is DANGER LESS!

    • fucsia profile image

      fucsia 7 years ago

      Very interesting informations! Thanks for sharing!

    • wrhapsody profile image

      wrhapsody 7 years ago from Renton, WA

      I cant figure out how to answer your question... but methadone will show up as an opiate in a standard drug test. You will fail and not be hired UNLESS you have a up-to-date prescription in your name for the medication.

    • profile image

      RaMa 7 years ago

      Fur(an)ocumarine are supposed to cause the enzymatic effect I watched on television - this info is missing here.

    • kimh039 profile image

      Kim Harris 7 years ago

      I like how you presented the information - broke it down into small parts. Nice read.

    • Hello, hello, profile image

      Hello, hello, 8 years ago from London, UK

      I never knew all that. Thanks for the information.

    • kre8iv4u profile image

      kre8iv4u 8 years ago from Florida beaches

      What a trip. My girlfriend eats a ton of grapefruit and out of nowhere a few months ago, broke out with hives. She finally figured it out, that some years ago, she had the same reaction while taking a medicine, at the same time eating grapefruit!!!



    • Research Analyst profile image

      Research Analyst 8 years ago

      Wow now this is an amazing find about how grapefruit juice can effect drug interactions, who knew!

    • fortunerep profile image

      fortunerep 8 years ago from North Carolina

      I just saw this warning on my medication today with is a benzo, not valium but xanax.  How uncanny this is and very important. Great Hub