Pen injection devices have acquired a pivotal role in insulin delivery, surpassing the use of conventional insulin syringes in many parts of the world. In this review we sought to determine differences in dosing accuracy between insulin pens and conventional syringes and vials, also touching on patient preference. We aggregated relevant literature found by searching the PubMed database, identifying seven relevant articles. There was consensus that pens are more accurate, especially at doses below 5 insulin units (IU). The literature also showed that pens tend to underdose when compared to syringes, but do this with a high degree of consistency. One study assessed influence on glycemic control, and whereas no significant difference was found with respect to hemoglobin A1c, fasting glucose levels decreased significantly more in pen users versus syringe users (-57 +/- 14 vs. 1 +/- 13 mg/dL, P = 0.003). The same study demonstrated that pens improved health-related quality of life compared to syringes and vials. We conclude that, regarding accuracy, there is sufficient evidence to recommend the use of insulin pens when delivering doses below 5 IU. For insulin doses above 5 IU there is no clear benefit for the pen in terms of accuracy.