Risk of prostate cancer death in long-term users of warfarin: a population-based case-control study.

PURPOSE: Recent evidence suggests that warfarin use may be associated with a reduced risk of prostate cancer. We aimed to determine whether exposure to warfarin is also associated with a reduced risk of prostate cancer death.
METHODS: A nested case-control study was conducted within a population-based cohort of 10,012 men aged ≥50 years with newly diagnosed prostate cancer between 1985 and 2002 and with no history of cancer since 1970 using the linked records of Saskatchewan Health and Saskatchewan Cancer Agency registry. We identified 2,309 cases who died of prostate cancer during follow-up. For each case, one control alive at the time of the case's death and matched for length of follow-up (±6 months) was randomly selected. Prescription counts were used to define warfarin exposure. Multivariate conditional logistic regression analysis was used to calculate the adjusted incidence rates of prostate cancer death in relation to warfarin use while adjusting for confounding by age, year of prostate cancer diagnosis, clinical stage and grade of cancer at diagnosis, Chronic Disease Score, and use of warfarin before diagnosis.
RESULTS: Ever use of warfarin following a diagnosis of prostate cancer was associated with an adjusted rate ratio of 1.44 (95 % confidence interval (CI) 1.33-1.84) for prostate cancer death. The adjusted rate ratio with one-year use of warfarin was 1.77 (95 % CI 1.25-2.50) compared to never use. The unadjusted rate ratio with five-year use of warfarin was 0.64 (95 % CI 0.40-1.00) and remained unchanged in the adjusted analysis (0.65, 95 % CI 0.37-1.13), although no longer statistically significant.
CONCLUSION: Our study does not provide conclusive evidence for a protective effect of long-term warfarin on prostate cancer-specific mortality. Moreover, short-term warfarin use may be associated with an increased risk of prostate cancer death.