Patients taking warfarin should avoid drinking cranberry juice, CSM warns
Patients treated with the anticoagulant warfarin should avoid drinking
cranberry juice, the Committee on Safety of Medicines has warned.
The CSM and the Medicines and Healthcare products Regulatory Agency highlight
a possible interaction between warfarin and cranberry juice in the September
issue of Current Problems in Pharmacovigilance. “The interaction
is biologically plausible since cranberry juice contains various antioxidants
including flavonoids, which are known to inhibit cytochrome P450 activity,” it
states.
The CSM also warns that constituents of different brands of cranberry
juice may vary, and that such variation might affect the potential for
drug interactions.
Dr Joanne Barnes, of the centre for pharmacognosy and phytotherapy, School
of Pharmacy, University of London, told The Journal that the suspected
interaction was a new signal of a herbal safety concern.
“Until further information is available, pharmacists should enquire
about use of cranberry juice in patients taking warfarin who experience
changes
in their INR [international normalised ratio] values.”
She pointed out that the MHRA is recommending that patients taking warfarin
should be advised to avoid drinking or limit their intake of cranberry
juice. “Along these lines, it would seem prudent to advise patients
taking warfarin and who consume large quantities of cranberry juice to
reduce their intake gradually, since the possible effect of concurrent
use of cranberry juice and warfarin on the INR, ie, an increase or decrease,
is not entirely clear.”
Since 1999, there have been five reports suggesting a possible interaction
between cranberry juice and warfarin, says the CSM. One case was fatal
and involved a man whose INR increased dramatically six weeks after starting
to drink cranberry juice.
Current Problems in Pharmacovigilance also includes information about
HRT and the risk of breast cancer, topical vaginal oestrogens and endometrial
safety, selective serotonin reuptake inhibitors and venlafaxine use in
children (see also p395), salmeterol and formoterol use in asthma management,
kava and hepatotoxicity, and the safety of thiomersal-containing vaccines.
The latest issue can be accessed here. |
