Aims/hypothesis: In the light of a report suggesting that insulin glargine may increase cancer occurrence, the EASD asked us to perform this study.
Methods: We followed 114,841 individuals who had a prescription dispensed for insulin between 1 July and 31 December 2005. From 1 January 2006 to 31 December 2007, we noted the occurrence of malignancies. Seven different nationwide registers were used to obtain information on insulin exposure, outcome and possible confounders; these were linked using the unique personal identity number assigned to every Swedish resident.
Results: After adjustment for age and, when appropriate, sex, users of insulin glargine alone (no other types of insulin), compared with users of types of insulin other than insulin glargine, had an RR of 1.99 (95% CI 1.31-3.03) for breast cancer, 0.93 (95% CI 0.61-1.40) for gastrointestinal cancer, 1.27 (95% CI 0.89-1.82) for prostate cancer and 1.07 (95% CI 0.91-1.27) for any type of malignancy. Adjustment for age, smoking, BMI, age at onset of diabetes, age at birth of first child, cardiovascular disease and oestrogen use gave an RR for breast cancer of 1.97 (95% CI 1.29-3.00). The 95% CIs crossed 1.0 for the RR calculated in all analyses of users of insulin glargine in combination with other types of insulin.
Conclusions/interpretation: In Sweden, during 2006 and 2007, women using insulin glargine alone (no other types of insulin) had an increased incidence rate of breast cancer as compared with women using types of insulin other than insulin glargine. This result may be due to a random fluctuation; the possibilities for examining validity are limited, and no statistically significant results were obtained for any other individual cancer site or for the outcome 'all malignancies'. No definitive conclusions regarding a possible causal relationship between insulin glargine use and the occurrence of malignancies can be drawn from the results of this study.