Trends in vitamin D supplement use in a general female and breast cancer population in Ireland: A repeated cross-sectional study

BACKGROUND: Vitamin D has been linked with improved survival after breast cancer diagnosis but little is known about prescribing rates. This study investigates trends in vitamin D supplement use in both a general female and breast cancer population. METHODS: Women with a breast cancer diagnosis were identified from the National Cancer Registry of Ireland (n = 19870). Women who had any vitamin D claim between 2005 and 2011 were identified from pharmacy claims data (n = 8556). Prevalence rates were calculated as a proportion of all eligible women and by age (< 55 years, ≥ 55 years). Poisson regression was used to compare rates of vitamin D prescribing across years (risk ratio (RR), 95% CI). RESULTS: There was a statistically significant increase in women with a claim for vitamin D between 2005–2011, with the largest increase among breast cancer patients aged ≥ 55 years (RR = 2.26; 95% CI, 2.11–2.42). CONCLUSION: This may have significant public health implications if associations between vitamin D and improved breast cancer survival prove to be causal.