Dec 18, 2012

By Trevor Shewfelt, Pharmacist at the Dauphin Clinic Pharmacy

I have a Houdini dog. Sheldon is a very sweet, but he is a bit of a chewer. He is fine when there are people in the house, but if he is left unsupervised, he chews things. So, when everyone is out we put him in a bathroom and blocked the door with a baby gate. That worked fine for a while. Then Sheldon figured out how to jump over the gate. So we started closing the sliding door to the bathroom. One night we came home from the kids hockey practice to find an excited and proud Sheldon running around the house. Apparently, Sheldon had discovered how to open the sliding door.


Discoveries like finding your dog can open a sliding door are often made by accident. In 1989, researchers, including David Bailey, a clinical pharmacologist at the Lawson Health Research Institute in London, ON, were looking at the effects of ethanol on the blood pressure pill felodipine. One of the things they had mixed the ethanol with to give it to test subjects was grapefruit juice. It turned out that the ethanol had no effect on the blood pressure pill, but the grapefruit juice did.


This grapefruit juice interaction-medication interaction was in the news again. The news article came to our attention via a phone call in the dispensary. One of the doctors called because they needed a list of all the drugs grapefruit juice interacted with. A patient had seen a report on the news and figured his grapefruit juice was going to kill him.


The interaction between grapefruit and medication has been talked about for over 20 years. I know it came up often when I was in pharmacy school. Grapefruit interactions with medications can be more concerning than other medication interactions because often I dont know you are taking grapefruit juice.


Here is how we usually look for medication interactions. In the pharmacy, we keep your medication list on a computer. We review your medications everytime you fill a prescription. Our computer system also scans your profile for interactions everytime you get a new prescription filled. It is a pretty good system. The combination of human eyes and computer checking catches most medication interactions, most of the time. But there are times we dont catch medication interactions. One type of potential interactions we miss is when you get your prescriptions filled at several different pharmacies. When that happens, I cant look at your medication profile and catch any interactions, and neither can my computer. Another potential for missing interactions is when you see several different doctors. Sometimes this is necessary, like when you have a family doctor and a specialist. However, when you see several doctors, the medications you are taking dont always get transferred perfectly between the physicians offices. One doctor could prescribe you a medication that interacts with a medication that she didnt know you were taking. If you see several different doctors, it is to your benefit to bring all those prescriptions to one pharmacy. That way that pharmacy can double check your profile for interactions.


Grapefruit juice is a different kind of potential interaction. The problem with grapefruit juice is that I dont know you are taking it. Even if I ask you, Do you eat grapefruit or drink grapefruit juice when you get a new prescription, you might answer no, because that week you didnt eat any grapefruit. Then 4 months later you might receive a box a grapefruit from a family member that spent the winter in Texas. You have forgotten that I asked about grapefruit 4 months earlier. I still dont know you are using grapefruit and you medication together. So catching grapefruit interactions is surprisingly tough.


What does grapefruit do to medications? Grapefruit juice inhibits an enzyme called cytochrome P450 3A4CYP3A4. This enzyme is found in the intestine and the liver. We think the grapefruit juice mainly affects the CYP3A4 in the intestine. Normally this enzyme breaks down the medication before it gets out of your intestine. For some medications, the CYP3A4 can break down 80% or more of the medication before it reaches the blood stream. So now if you drink a glass of grapefruit juice with your medication, the grapefruit juice inhibits this enzyme, and less of the drug is broken down in the intestine. This means more of the drug is absorbed. If the drug was normally heavily broken down by the enzyme, taking 1 pill of that medication with grapefruit juice could be like taking 20 pills with water. For example if a patient takes a blood pressure pill that is affected by grapefruit juice, it might now be like taking 20 pills of the blood pressure medication. This might lower the patients blood pressure too much. Spacing the medication away from the grapefruit juice wont help either. The grapefruit juice can affect the enzymes for up to 24 hours.


What medications are effected by grapefruit juice? In an article published in the Canadian Medical Association Journal in November 2012, David Bailey et. al estimated that there were at least 80 drugs that interacted with grapefruit juice. One example is atorvastatin. Atorvastatin is a common cholesterol pill. Taking grapefruit juice and atorvastatin can cause you to get too much atorvastatin in your system. That can increase your risk of getting damaged muscles and kidney problems, a condition called rhabdomyolysis.


Some discoveries you just dont want to make. I didnt want to discover my dog Sheldon on the dining room table chewing on my pizza. You dont want to stumble on a grapefruit medication interaction either. If you take medications, before eating grapefruit or drinking its juice, call your pharmacist. Ask us if there are any interactions. With some good communication between the public and their pharmacists, we can keep everyone safe.

As always if you have any questions or concerns about these or other products, ask your pharmacist


CMAJ article:

CBC story on Grapefruit juice:


The information in this article is intended as a helpful guide only. It is not intended to be used as a substitute for professional advice. If you have any questions about your medications and what is right for you see your doctor, pharmacist or other health care professional.


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