When you receive a prescription from a GP or community pharmacist, a lot of considerations go into ensuring it is safe, effective and fit for purpose.
This includes checking your medical history, any potential allergies or intolerances, and being mindful of other medications you are taking, in case there are any reports of adverse interactions between the new drug and any you are already taking.
For example, whilst some over-the-counter painkillers can be taken together, aspirin and ibuprofen should not be taken together because they can increase the risk of stomach bleeding and kidney damage.
However, it is not just other medications that can interact with each other but also some foods and drinks, which can cause drugs to either not work as effectively or increase the risk of an overdose effect despite taking the right amount.
This is why, if you take the anticoagulant warfarin, you need to avoid avocados and leafy green vegetables such as spinach and broccoli, which can complicate meal planning for people taking the medicine for high blood pressure.
One of the most common examples of a food or drink interacting with medication is grapefruit juice, to the point that a label warning that you should not take a medication with grapefruit is something that a lot of people have seen, including people who take warfarin.
Exploring why this is the case can also help explain other interactions and provide safer ways to eat what is otherwise a relatively healthy citrus fruit.
What Makes Grapefruit Harmful With Some Medications?
According to the National Institute of Health and Care Excellence, at least 100 medications interact with grapefruit juice in some way, to the point that the fruit has its own page simply so its interactions can be listed even though it otherwise has no clinical use.
The first discoveries about grapefruit juice were made by accident in 1989 when performing a blind test of certain drug interactions.
Typically 200ml or the equivalent of a whole grapefruit is enough to cause issues, but it is important to understand why this is the case to ensure you avoid any major issues.
Whilst there are some strange and unusual exceptions to this, there are two main ways in which grapefruit interacts with medication.
The first is that it acts as an inhibitor of certain enzymes, stopping certain drugs from being metabolised properly. As dosages are designed based on typical levels of absorption, this can lead to a much higher dosage of a drug than expected in the body.
This is part of the reason why it is dangerous to take grapefruit juice with warfarin, but it also affects statins, benzodiazepines and some antiretrovirals such as ritonavir.
Conversely, it can also affect medications by blocking the transporter cells that move the
compound from the intestines into the blood.
This has the complete opposite effect of enzyme blocks in that it means that not enough of the drug is in the body, stopping its effectiveness and potentially causing major problems.
The most concerning example of this is clopidogrel, an antiplatelet medicine that reduces stroke and heart attack risk. This medication not working effectively could put people in danger so grapefruit should be avoided.