Insulin is a high-risk medication, and even slight changes in blood levels can lead to serious side effects or can even result in death. Error in administering drugs is one of the main causes of over- or under-dosing, and the recent introduction of concentrated insulins (CI) has increased this risk. We assessed nurses’ knowledge of these CI, their beliefs about the “insulin unit” (IU), and the impact that this knowledge had on the risk of making medication errors. A direct interview survey was conducted in eight departments of medicine and surgery in a university hospital. Sixty-eight nurses and midwives were interviewed. Twenty-six percent of them had already encountered a CI prescription and only 51.5 percent correctly defined the notion of IU. Only 18 percent responded correctly to a practical case of a CI prescription, whilst 35 percent multiplied the dose by two and 24 percent divided it by two. Sixty percent indicated that they regularly use a U-100 graduated insulin syringe to withdraw insulin from the pen. Insulin administration errors related to this misuse, which are very well documented in the literature, are linked to nurses’ lack of knowledge about the true definition of IU. These administration errors have increased with the introduction of concentrated insulins.