There have been growing concerns regarding the long-term effects of insulin glargine (A21Gly,B31Arg,B32Arg human insulin) on the risk of breast cancer.

We used the UK’s General Practice Research Database (GPRD) to identify a cohort of women aged 40 years or over with type 2 diabetes, treated with insulin during 2002–2006 and followed until the first breast cancer diagnosis or 31 December 2009. After the users of insulin glargine had been matched with users of other insulins on age, calendar time and duration of prior insulin use, the HR of breast cancer associated with insulin glargine use was estimated using a Cox proportional hazards model, adjusted for known risk factors for breast cancer.

The cohort comprised 15,227 women, including 4,579 glargine users and 10,648 users of other insulins, of which 246 developed breast cancer during up to 8 years follow-up (incidence rate 4.1 per 1,000 per year). Insulin glargine use was not associated with an increased risk of breast cancer during the first 5 years of use (HR 0.9; 95% CI 0.7–1.3). The risk tended to increase after 5 years (HR 1.8; 95% CI 0.8–4.0), and significantly so for the women who had been on insulin before starting glargine (HR 2.7; 95% CI 1.1–6.5).

The risk of breast cancer in women with type 2 diabetes is not increased during the first 5 years of insulin glargine use. However, longer-term use may increase this risk, particularly in women with longstanding use of insulin before starting insulin glargine.