Nonadherence to adjuvant tamoxifen therapy in women with primary breast cancer.

PURPOSE Although clinical trials have clearly demonstrated the benefits of tamoxifen in women with primary breast cancer, little is known about how this drug is actually used in the general population. We sought to estimate adherence and predictors of nonadherence in women starting tamoxifen as adjuvant breast cancer therapy. PATIENTS AND METHODS Subjects were age 18 years or older initiating tamoxifen for primary breast cancer and enrolled in New Jersey's Medicaid or Pharmaceutical Assistance to the Aged and Disabled programs during the study period, from 1990 to 1996 (N = 2,378). Main outcome measures were number of days covered by filled prescriptions for tamoxifen in the first year of therapy with the 4 years after tamoxifen initiation for a subset; predictors of good versus poor adherence. RESULTS Twenty-three percent of patients missed taking tamoxifen on more than one fifth of days studied, although on average, patients filled prescriptions for tamoxifen for 87% of their first year of treatment. The youngest, oldest, nonwhite, and mastectomy patients had significantly lower rates of adherence; patients who had seen an oncologist before taking tamoxifen had significantly higher rates of adherence. Overall adherence decreased to 50% by year 4 of therapy. CONCLUSION The mean level of adherence to tamoxifen is high compared with other chronic medications. However, nearly one fourth of patients may be at risk for inadequate clinical response because of poor adherence. Because of the efficacy of tamoxifen therapy in preventing recurrence and death in women with early-stage breast cancer, further efforts are necessary to identify and prevent suboptimal adherence.