Adherence in patients transferred from immediate release metformin to a sustained release formulation: a population-based study

Metformin is the most commonly prescribed oral agent used in the treatment of type 2 diabetes. It is effective at reducing glycosylated Haemoglobin (HbA1c) and decreasing microvascular and macrovascular disease. However, up to 25% of patients develop gastrointestinal side effects leading to cessation in 5-10% of users. Metformin XL (glucophage SR) is a once a day preparation that delays absorption, leading to decreased peak metformin concentrations. We hypothesised that the XL preparation of metformin would be better tolerated than the standard immediate release (IR) preparation leading to improved adherence to therapy. In a retrospective observational study, we studied adherence and glycaemic control in patients prescribed metformin IR and XL preparations in Tayside, UK. Metformin XL was used by 137 patients during the study period. Overall adherence was greater in the XL group (80%) compared with the 10 772 patients in the IR group (72%, p = 0.0026). In the 40 patients who changed from metformin IR to metformin XL who had sufficient data to determine adherence, the adherence increased from 62% in the IR group to 81% in the XL group (p < 0.0001). This was associated with an HbA1c reduction from 9.1 to 8.4% (p = 0.0739, n = 29). Metformin XL use is associated with increased adherence compared with the IR preparation, although the mechanism for this cannot be determined from this study. In patients intolerant of metformin IR the XL preparation should be considered.