Infographic on the chemistry of grapefruit. Grapefruit and grapefruit juice interact with a large number of drugs, usually resulting in adverse effects. These interactions are caused by a class of compounds called furanocoumarins, in particular the compounds bergamottin and dihydroxybergamottin. Furanocoumarins inhibit some forms of an enzyme, CYP3A4, responsible for breaking down drugs in the body. As the prescribed doses of drugs take into account how quickly the drug is broken down in the body, this can cause higher concentrations of the drug in the bloodstream, which in turn can result in unpleasant side effects. The colour of pink and red grapefruits is caused by the compound lycopene.
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You may have heard of ‘the grapefruit juice effect’ – a range of medications recommend avoiding grapefruits or grapefruit juice whilst you are taking them, due the unfavourable interactions with the medication and the unwanted side effects this can cause. So, what chemical compounds cause these interactions?

The principal culprits are a family of chemical compounds called furanocoumarins, and particularly the compounds bergamottin & dihydroxybergamottin. Both of these compounds interfere with the activity of an enzyme that plays an important part in breaking down some drugs in the body, and prevent it from doing so. This, in turn, can lead to increased levels of concentration of the drug in the bloodstream. This is a problem, because prescriptions for drugs take into account the rate at which your body breaks down the drug in their dosage recommendations. Since these compounds in grapefruits greatly decrease the rate of breakdown, repeated doses can lead to much higher doses of the drug in the bloodstream, in turn potentially leading to harmful side effects.

This effect of grapefruit juice is also long lasting, with it taking around 24 hours for the enzyme’s activity to recover to half of its original level, and full recovery taking up to 72 hours. One whole grapefruit, or 200ml of juice, can be enough to cause significant interaction with enzyme activity. The side effects can vary depending on the drug being taken, but can potentially include kidney damage, blood clots, and breakdown of muscle fibres.

The pomelo (the principal ancestor of the grapefruit) exhibits similar interactions with the enzyme; however, the tangelo, a fruit which is a cross between a tangerine or pomelo and a grapefruit, contains only trace amounts of bergamottin, and as such will not interact with the enzyme, and can be consumed safely with the same drugs that grapefruit would interfere with.

As an interesting aside, grapefruits negative side effects on drugs have also been investigated for their potential benefits. Several anti-AIDs drugs, which would otherwise be broken down in the body quite quickly, could have their usefulness extended by being taken in combination with grapefruit, which would extend the amount of time the drug is present in the bloodstream.

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