Possible interaction between warfarin and cranberry juice

Suvarna and colleagues reported the possible interaction between
warfarin and cranberry juice (1). We remain unconvinced and feel that the
case for an interaction is unproven.

Their report concerned a man who “ate next to nothing” and took only
cranberry juice for 6 weeks. The fact that he presented with major
haemorrhage and an INR of >50 may have nothing to do with the cranberry
juice. A much more likely explanation is the fact that he, presumably,
continued on the same dose of warfarin despite no vitamin K intake from
his food. Any patient who stops eating for 6 weeks but takes the same dose
of warfarin is likely to become over-anticoagulated.

Of the three other patients referred to, one had a decrease in their
INR a conclusion difficult to explain by the authors’ proposed hypothesis.
The other two patients had unstable INRs but this is not a rare phenomenon
in anticoagulation clinics. At any one time, even in good anticoagulation
clinics, it is rare to have >70% of the patients within the therapeutic
range.

Approximately 1% of the UK population is on warfarin and cranberry
juice intake is increasing in popularity. By chance alone one can expect
many patients taking warfarin and cranberry juice to be over-
anticoagulated at any one time.

The Committee on Safety of Medicines (CSM), based on these 4 cases
issued a warning about a possible interaction between warfarin and
cranberry juice (2). It is important that the CSM make public the full
data on which they gave this advice. Based on what we have seen, the case
for an interaction remains unproven.

References:

1) Suvarna R, Pirmohamed M, Henderson L. Possible interaction between
warfarin and cranberry juice. BMJ 2003; 327:1454

2) Committee on Safety of Medicines. Possible interaction between warfarin
and cranberry juice. Current Problems in Pharmacovigilance 2003; 29:8

Competing interests:
None declared