Hi, I m sorry to hear you ve had a lot of hair loss while using Fentanyl. Though uncommon, hair loss is a known side effect of Fentanyl. I would recommend stopping the Fentanyl and switching to another medication if possible. I don t think it happens enough to know exactly how your body would respond, but if you keep taking Fentanyl, I would expect that you will continue to lose hair, and it may worsen. If you stop I think there is a good chance your hair will recover as normal. Hope I have answered your query. Let me know if I can assist you further.