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TORONTO, CANADA. In the August 2000 issue of IHN we reported on the findings of Dr. Michelle
Cotterchio and colleagues (New England Journal of Medicine, Vol. 342, June 29, 2000, p. 2003) to
the effect that the use of the antidepressant paroxetine (Paxil) may increase the risk of breast cancer by a
factor of seven. If confirmed these findings would almost certainly lead to a caution against prescribing
paroxetine to women. Not surprisingly, Dr. Cotterchio's report has caused a considerable stir within the
medical and pharmaceutical communities. The December issue of the American Journal of Epidemiology
contains two letters (one from the manufacturer of paroxetine, SmithKline Beecham Pharmaceuticals) that
question the validity of Dr. Cotterchio's conclusions.
The authors of both letters point out that the number of cases of possible paroxetine-related breast cancer
was small (9 cases and 1 control) and the SmithKline Beecham letter also makes the observation that most
of the patients who had taken paroxetine had previously been exposed to fluoxetine (Prozac). Dr.
Cotterchio replies that her report concluded that "use of paroxetine may be associated with a substantial
increase in breast cancer" and that future studies would be required in order to confirm this. She also
reiterates that, while she and her team do not suggest that paroxetine is a carcinogen (cancer initiator), they
do believe that it could well be a promoter of breast cancer. She points out that animal studies support the
hypothesis that antidepressants may be tumor promoters. It would seem that the final conclusion on the
possible relationship between paroxetine and breast cancer must await the completion of the larger
study.
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