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Introduction: Fibromyalgia causes long-term pain. It affects at least 2% of the population, the majority being women. In addition, extended symptoms corresponding to vitamin B 12 deficiency occur. Findings from several studies have indicated that vitamin B 12 may be a possible treatment for pain in fibromyalgia. The aim of the proposed study is to evaluate whether vitamin B 12 decreases pain sensitivity and the experience of pain (ie, hyperalgesia and allodynia) in women with fibromyalgia., Methods and Analysis: The study is a randomised, placebo-controlled, single-blind, clinical trial with two parallel groups which are administered mecobalamin (vitamin B 12 ) or placebo over 12 weeks. 40 Swedish women aged 20-70 years with an earlier recorded diagnosis of fibromyalgia are randomised into the placebo group or the treatment group, each consisting of 20 participants. Outcomes consist of questionnaires measured at baseline and after 12 weeks of treatment. A final re-evaluation will then follow 12 weeks after treatment ends. The primary outcome is tolerance time, maximised to 3 min, which is assessed using the cold pressor test. In order to broaden the understanding of the lived experience of participants, qualitative interviews will be conducted using a phenomenological approach on a lifeworld theoretical basis (reflective lifeworld research approach)., Ethics and Dissemination: The protocol for the study is approved by the local ethical committee at Linkoping (EPM; 2018/294-31, appendices 2019-00347 and 2020-04482). The principles of the Helsinki Declaration are followed regarding oral and written consent to participate, confidentiality and the possibility to withdraw participation from the study at any time. The results will primarily be communicated through peer-reviewed journals and conferences., Trial Registration Number: NCT05008042., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)