<b><i>Background and Aims:</i></b> For many years, flunitrazepam was the benzodiazepine of choice among users of illegal drugs. The aim of this study was to investigate to which extent clonazepam use has increased in this population, and whether this was related to increased prescription or because of illegal availability. <b><i>Methods:</i></b> We used data from three sources to study the changes in the use of clonazepam: (1) Presence and concentrations of clonazepam and flunitrazepam in blood samples collected from Norwegian drugged drivers; (2) Sales numbers (legal market) for clonazepam, extracted from the Norwegian prescription database (NorPD), and (3) Specific seizures (illegal market) for clonazepam in Norway. <b><i>Results:</i></b> In 2004, 13.0% of the analysed blood samples from drugged drivers contained clonazepam, whereas this proportion had increased to 27.7% in 2013. In the same period, the frequency of flunitrazepam in drugged drivers decreased from 16.6% in 2004 to 3.2% in 2013. The number of clonazepam prescriptions decreased, while the number of seized tablets containing clonazepam increased considerably from 2004 to 2013. <b><i>Conclusions:</i></b> For the last 10 years, a significant increase in the illegal use of clonazepam has been seen, now replacing flunitrazepam as the most used illegal benzodiazepine in Norway.