Purpose: To demonstrate the usefulness of hair testing to determine exposure pattern to fentanyls. Methods: A 43-year-old male was found unconscious with respiratory depression 15 min after snorting 3 mg of a powder labeled as butyrylfentanyl. He was discharged from hospital within 2 days without blood or urine testing. Two locks of hair were sampled 1 month (M1 A: 0–2 cm (overdose time frame); B: 2–4 cm; C: 4–6 cm) and 1 year (Y1: A: 0–2 cm; B: 2–4 cm) later to monitor his exposure to drugs of abuse by liquid chromatography–tandem mass spectrometry after liquid-liquid extraction. Results: Hair analysis at M1 showed a repetitive exposure to 3-fluorofentanyl (A/B/C: 150/80/60 pg/mg) with higher concentration in segment A reflecting the overdose period. The non-detection of butyrylfentanyl was consistent with the analysis of the recovered powder identified as 3-fluorofentanyl. Furanylfentanyl (40/20/15 pg/mg) and fentanyl (37/25/3 pg/mg) were also detected in hair. The second hair analysis at Y1 showed the use of three new fentanyls, with probably repetitive exposures to methoxyacetylfentanyl (A/B: 500/600 pg/mg), and single or few exposures to carfentanil (2.5/3 pg/mg) and acetyl fentanyl (1/1 pg/mg). A decreasing exposure to 3-fluorofentanyl (25/80 pg/mg), and increasing consumption of furanylfentanyl (310/500 pg/mg) and fentanyl (620/760 pg/mg) were also observed despite methadone treatment initiation. The patient claimed not consuming three out of the six detected fentanyls. Conclusions: We report single or repetitive exposure to several fentanyls using hair testing. To our knowledge, this is the first demonstration of 3-fluorofentanyl and methoxyacetylfentanyl in hair samples collected from an authentic abuser. © 2019, The Author(s).