ABSTRACT Introduction Fentanyl test strips (FTS) are increasingly recommended for non‐heroin drugs to detect potential fentanyl adulteration. The aim of this study was to better understand how people are using FTS. Methods In simulation exercises between September and October 2023, 40 people who used drugs participated in a mock use of FTS on a simulated drug, interpreted FTS results and completed interviews about their FTS use. Data were analysed in NVivo. Results Prior to study enrolment, 80% of participants reported receiving training on using FTS, some had instructed others on their use, and 71% reported a positive result at the last use of a FTS on a non‐heroin drug. During the simulation exercise, none of the participants used FTS as recommended: most under‐diluted the sample or used the FTS in another manner not indicated. During the interpretation of FTS strip results, 45% correctly interpreted a positive test; 55% a negative test with clear lines; and 30% a negative test with a faint second line. Discussion and Conclusions Observed errors in FTS simulations would likely lead to false positive results. Findings highlight the lack of appropriate training people have received and the need to develop educational approaches to ensure people use FTS properly to optimise their impact.