“A breakfast of grapefruit and marmalade on toast could be lethal for people taking medication” is the headline in the Daily Mail.
The news is based on a review that has highlighted grapefruit-medication interactions that can cause serious side effects.
It is already known that grapefruits contain a group of chemicals, furanocoumarins, which can affect drug metabolism – the amount of time it takes for a drug to be broken down by the body.
The chemical inhibits an enzyme that breaks down drugs, this can cause more ‘active’ drug to be present in the body than was intended with the given dose. This can then trigger unpleasant, and sometimes serious, side effects.
However, the authors state that there is a general lack of knowledge about this interaction in the healthcare community, despite the fact that an interaction between grapefruit and certain medications was discovered more than 20 years ago.
The number of drugs that could potentially interact with grapefruit to cause serious adverse events has been increasing. The researchers say that there are now 43 types of prescription medication that could cause serious side effects if taken with grapefruit (or grapefruit juice).
Researchers from Canada have published a narrative review (a review that discusses and summarises the literature on a particular topic) of grapefruit-medication interactions in the peer-reviewed Canadian Medical Association Journal.
Grapefruit contains a class of chemicals known to inhibit the process by which a number of drugs are broken down by the body (inactivated).
This causes the concentration of drugs in the body to be higher than it should be, which can cause side effects.
The researchers state that more than 85 drugs available in Canada have the possibility of interacting with grapefruit. The number of drugs which could potentially interact with grapefruit and lead to serious side effects has recently increased to 43 as new drugs come on the market.
While not addressed by the researchers, it is likely that broadly similar figures would apply to the UK market.
The authors report that grapefruit, grapefruit juice, and some other citrus fruit, including Seville oranges, limes and pomelos, contain a class of chemical called furanocoumarins. Furanocoumarins inhibit an enzyme, cytochrome P450 3A4, that is responsible for the inactivation of approximately half of all drugs.
This enzyme is found in the lining of the gut and in the liver. Furanocoumarins in grapefruit mainly inhibit cytochrome P450 3A4 in the gut.
This means that if a drug that is normally broken down by cytochrome P450 3A4 is taken at the same time as grapefruit, more ‘active’ drug will be absorbed by the body, as less will have been inactivated by the enzyme.
The prescribed dose of these drugs takes into account the fact that some of the drug will be inactivated by P450 3A4. So if this doesn’t occur, it leads to the person being exposed to higher concentrations of the drug than was intended, and this can have adverse effects.
Furanocoumarins are not present in varieties of sweet orange, such as naval or Valencia oranges.
Read more about how grapefruits can affect your medication.
The researchers report that furanocoumarins are present in all forms of grapefruit (freshly squeezed juice, frozen concentrate and whole fruit).
One whole grapefruit or 200ml of juice is sufficient to cause enough of an increase in the concentrations of active drugs to have an effect on the body, and therefore could cause side effects.
The time between consuming grapefruit and taking the medication, and the frequency of consumption of grapefruit, can also influence their effect.
The researchers recommend that it is better to err on the side of caution and never have any grapefruit (or other citrus fruit containing furanocoumarins) when taking drugs known to interact with these types of fruit.
Drugs that interact with grapefruit are:
The researchers provide a list of drugs that are predicted to interact with grapefruit, including:
The researchers identified the following examples of cases of potentially serious adverse events that have been reported in published literature as a result of grapefruit-drug interactions:
What makes an individual person more vulnerable to interactions between grapefruit and their medications is not well understood, but it may depend on the:
The simple answer to this is that nobody is really sure. The researchers state that data is not available to be able to estimate how common grapefruit-drug interactions are in routine practice.
They say that this is because there is a lack of knowledge about grapefruit-medication interactions in the healthcare community, resulting in under-reporting of these interactions.
In the UK, adverse reactions to medication are monitored by the MHRA’s Yellow Card Scheme, but currently, the monitoring system used by the scheme does not collect information on grapefruit consumption.
Also, the researchers did not comment on how frequently grapefruit-medication interactions resulted in death.