Aims: To identify real-world factors affecting adherence to insulin therapy in patients with Type 1 or Type 2 diabetes mellitus.

Methods: A literature search was conducted in PubMed and EMBASE in November 2011 to identify studies reporting factors associated with adherence/non-adherence to insulin therapy in adults with Type 1 or Type 2 diabetes.

Results: Seventeen studies were identified; six used self-reported measures and 11 used calculated measures of adherence. Most (13/17) were conducted exclusively in the USA. Four categories of factors associated with non-adherence were identified: predictive factors for non-adherence, patient-perceived barriers to adherence, type of delivery device and cost of medication. For predictive factors and patient-perceived barriers, only age, female sex and travelling were associated with non-adherence in more than one study. Fear of injections and embarrassment of injecting in public were also cited as reasons for non-adherence. Conversely, adherence was improved by initiating therapy with, or switching to, a pen device (in four studies), and by changing to an insurance scheme that lowered the financial burden on patients (in two studies).

Conclusions: Adherence to insulin therapy is generally poor. Few factors or patient-perceived barriers were consistently identified as predictive for non-adherence, although findings collectively suggest that a more flexible regimen may improve adherence. Switching to a pen device and reducing patient co-payments appear to improve adherence. Further real-world studies are warranted, especially in countries other than the USA, to identify factors associated with non-adherence and enable development of strategies to improve adherence to insulin therapy.