To meet the WHO vision of reducing medication errors by 50%, it is essential to know the current error rate. We undertook an integrative review of the literature, using a systematic search strategy. We included studies that provided an estimate of error rate (i.e. both numerator and denominator data), regardless of type of study (e.g. RCT or observational study). Under each method type, we categorised the error rate by type, by classification used by the primary studies (e.g. wrong drug, wrong dose, wrong time), and then pooled numerator and denominator data across studies to obtain an aggregate error rate for each method type. We included a total of 30 studies in this review. Of these, two studies were national audit projects containing relevant data, and for 28 studies we identified five discrete method types: retrospective recall (6), self-reporting (7), observational (5), large databases (7), and observing for drug calculation errors (3). Of these 28 studies we included 22 for a numerical analysis and used six to inform a narrative review. Drug error is recalled by~1 in 5 anaesthetists as something that happened over their career; in self-reports there is an admitted rate of~1 in 200 anaesthetics. In observed practice, error is seen in almost every anaesthetic. In large databases, drug error constitutes~10% of anaesthesia incidents reported. Wrong drug or dose form the most common type of error across all five study method types (especially dosing error in paediatric studies). We conclude that medication error is common in anaesthetic practice, although we were uncertain of the precise frequency or extent of harm. Studies concerning medication error are very heterogenous, and we recommend consideration of standardised reporting as in other research domains.