The market for new psychoactive substances has increased markedly in recent years and there is now a steady stream of compounds appearing every year. Benzodiazepines consist of only a fraction of the total number of these compounds but their use and misuse has rapidly increased. Some of these benzodiazepines have only been patented, some of them have not been previously synthesised, and the majority have never undergone clinical trials or tests. Despite their structural and chemical similarity, large differences exist between the benzodiazepines in their pharmacokinetic parameters and metabolic pathways and so they are not easily comparable. As benzodiazepines have been clinically used since the 1960s, many analytical methods exist to quantify them in a variety of biological matrices and it is expected that these methods would also be suitable for the detection of benzodiazepines that are novel psychoactive substances. Illicitly obtained benzodiazepines have been found to contain a wide range of compounds such as opiates which presents a problem since the use of them in conjunction with each other can lead to respiratory depression and death. This review collates the available information on these benzodiazepines and provides a starting point for the further investigation of their pharmacokinetics which is clearly required.