Recent metformin adherence and the risk of hypoglycaemia in the year following intensification with a sulfonylurea

Aim To evaluate whether recent low adherence to metformin monotherapy is associated with hypoglycaemia after addition of a sulfonylurea. Methods We assembled a retrospective cohort of veterans who filled a new prescription for metformin between 2001 and 2011 and intensified treatment with a sulfonylurea after ≥1 year of metformin use. We calculated metformin adherence from pharmacy data using the proportion of days covered in the 180‐day period before intensification. The primary outcome was hypoglycaemia, defined as a hospitalization or emergency department visit for hypoglycaemia or an outpatient blood glucose measurement <3.3 mmol/l in the year following intensification. Cox proportional hazards models were used to compare the risk of hypoglycaemia between participants with low (<80%) and high (≥80%) adherence. Adherence was also modelled as a continuous variable using restricted cubic splines. Results Of 187 267 participants who initiated metformin monotherapy, 49 424 added a sulfonylurea after ≥1 year. The median (interquartile range) rate of treatment adherence was 87 (50–100)% and 43% had adherence <80%. Hypoglycaemia rates per 1000 person‐years were 23.1 (95% CI 21.1–25.4) and 24.5 (95% CI 22.7–26.4) in participants with low and high adherence, respectively (adjusted hazard ratio 0.95, 95% CI 0.84–1.08). The risk of hypoglycaemia was similar across all levels of adherence when adherence was modelled as a continuous variable. Conclusions We found no evidence that past low adherence to metformin monotherapy was associated with hypoglycaemia after intensification with a sulfonylurea.