This bulletin provides new data on the prevalence of fentanyl use among a sample of Australian police detainees. A survey of 1,044 detainees conducted under the Drug Use Monitoring in Australia program in 2018 found that three percent of detainees reported having used fentanyl in the past 12 months. The same proportion of detainees interviewed in 2016 reported having used the drug. The most common reason that detainees started to use fentanyl was that it had been prescribed for medical reasons. Seventy-one percent of frequent users had engaged in non-medical use of fentanyl. Most of these users were given fentanyl by their family and friends.