Inclusion Criteria:  Women aged 20-70 years Swedish-speaking Diagnosis of fibromyalgia Safe method of contraception Cobalamin / p (vitamin B12) should be >250 pmol/L < 800 pmol/L Given consent to participate.  Exclusion Criteria:  Previous treatment with B12 Known hypersensitivity to the active substance Mecobalamin or an additive Vegan as veganism can lead to B12 deficiency Neuroleptics Known heart, kidney or liver disease Reynaud's phenomenon (white fingers) Known neuropathy (impaired sensation) Breastfeeding Pregnancy or plan pregnancy