Grapefruit

How it works

Constituents such as furanocoumarins irreversibly inhibit intestinal cytochrome P450 3A4, reducing first-pass metabolism and increasing the bioavailability and plasma concentrations of many CYP3A4 substrate drugs.

Prescribing in practice

• Grapefruit can substantially raise levels of numerous drugs (for example certain statins, some calcium-channel blockers and immunosuppressants), increasing toxicity, so check interaction status and advise avoidance where relevant.
• The effect can persist for some time after consumption, so simply separating intake does not reliably avoid the interaction.
• Consider grapefruit exposure when investigating unexpected adverse effects or fluctuating drug levels.

Monitoring

Where interacting drugs cannot be avoided, monitor for enhanced drug effect or toxicity and, for relevant agents, drug levels.

Counselling the patient

• Avoid grapefruit and grapefruit juice if your medicine interacts with it, as advised.
• Tell your pharmacist or prescriber about your usual diet when starting new medicines.

Evidence & guidelines

Grapefruit-juice inhibition of intestinal CYP3A4 and its clinical consequences are well documented in pharmacology literature and product information.