Aims/hypothesis: To further investigate the association of cancer occurrence with the use of insulin glargine.
Methods: We followed 114 838 individuals using insulin between 1 July and 31 December 2005. From 1 January 2006 to 31 December 2008, we noted the occurrence of malignancies (cohort I). Insulin users between 1 July and 31 December 2006 were followed for the occurrence of malignancies in 2007 and 2008 (cohort II). Users of insulin during three consecutive six-month periods from 1 July 2005 to 31 December 2006 were followed for the occurrence of malignancies in 2007 and 2008 (cohort III). The Prescribed Drug Register, the Cancer Register, and the Causes of Death Register were used to obtain information on targeted person-time and outcome. We retrieved variables reflecting potential confounding factors from the Swedish National Diabetes Register, the Prescribed Drug Register, the Patient Register, the Medical Birth Register and the National Education Register. With Poisson regression we evaluated the association between insulin use and malignancy outcome with adjustment for confounders.
Results: The adjusted incidence rate ratio (and 95% confidence interval) for women who used insulin glargine alone compared with those who used other types of insulin, was 1.60 (1.10-2.32) for breast cancer but included 1.0 for malignancy outcomes other than breast cancer for men and women when analyzing cohort I with follow-up in 2006-2008. For cohort II and III the corresponding incidence rate ratios were 1.38 (0.87-2.18), and 0.87 (0.41-1.85), respectively.
Conclusion/interpretation: We do not see an increased risk during 2008 for breast cancer in the insulin glargine group. We need data for additional years before we can state with reasonable certainty that the increase in breast cancer incidence that we observed in Sweden in 2006 and 2007 was due to a random fluctuation or whether there is an association with the use of insulin glargine.