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Drug Interactions and The Grapefruit Juice Effect

Have you ever read the label of your medicines and saw ‘Avoid Grapefruit Juice While Taking This Medicine’, and wondered why? Well, as you may be aware, drugs can interact with other drugs, but they can also interact with vitamins, minerals, supplements, and even foods and drinks! To give an example of a common interaction with medication, which you may be familiar with, I will explain the Grapefruit Effect, and use this to explain some of the other interactions you come across also, and how important it is to avoid them!

The Grapefruit Effect - Why to avoid grapefruit when it says so on your drug label

So, you may think having some grapefruit or grapefruit juice is a great way to start your day and to get your vitamins in, however, the health benefits are outweighed by the interaction it can have with your medications, if this is applicable to you. There is a class of compounds in grapefruits called furanocoumarins. This may sound like a fancy chemical name, but basically they are just a class of substance in grapefruits and other foods which have similarities and similar effects. These substances can interact with a host of different drugs, leading to either a higher or lower level of the drug in your body, depending on the drug. These furanocoumarins interact with a class of enzymes in your liver, which normally help to break down and metabolise drugs, or in some circumstances, convert the drug from its inactive form to its active form.

About 50% of drugs are metabolised (broken down) by these enzymes in the liver (Cytochrome P450 – 3A4), so if your grapefruit juice is blocking these enzymes, the drugs you are taking will not be broken down like they should be, so more of the drug is in the body than is desired/predicted so it is almost like you have overdosed, even though you took the same amount of tablets at the same dosage as you usually would. If the drug is the type of drug that needs to be activated rather than inactivated by these enzymes in the liver, then the result of having grapefruit juice with them would be that the drug is not activated and you get lower levels of active drug in the body than expected, as would be the case with the antidepressant drug sertraline.

The inhibition of these enzymes in the liver is irreversible, therefore in order to get your drug metabolism back to normal, your body needs time to make more of these enzymes, which explains why the effect of consuming grapefruit has a longer lasting effect than just the amount of time it is in the body for (effects can last up to 24 hours!).

Examples of drugs which are affected by this interaction include paracetamol, sertraline, warfarin, Viagra, morphine, codeine and many others.

The effect of this interaction, when it increases levels of drug in the body, can be like the effects of overdose, and can lead to significant side effects requiring hospitalisation. Not to scare you, just to highlight the importance of reading the label on your drugs properly and following the advice your healthcare practitioner has given you (doctor, dentist, pharmacist). The basis of this interaction that I have briefly explained, is the basis for many other interactions – drugs and food and supplements often compete for the same enzymes to break them down and digest them in the liver, and with all this competition, this can make the drug (active or inactive) hang around in the body for longer than desired, and tgis can have some nasty effects if we do not watch out for it.

Looking out for interactions is all part of the pharmacist’s job, which is why whenever you get medications in the pharmacy, even over the counter medications, your pharmacist or the counter assistant will ask you if you are taking any other medications, and it is important that you disclose these as you never know what interactions may exist and cause more harm than good. It is better to be safe than sorry!

That’s all for now, but I hope this was helpful/a little bit interesting for any nerds out there like me. As always, send me questions if you have any ☺️


Lauren x

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