Patients Study participants were screened from a pool of patients with chronic treatment resistant neuropathic pain and were eligible to participate in the study after giving written informed consent Study design The patients visited the Pain Clinic twice. Oral and written information about the study was provided and informed consent obtained. Demographic data (date of birth, sex, and ethnic background, medical and surgical history) were recorded. Information about the patients' assessments were recorded before the injection, including current medication and other (successful or non-successful) treatment attempts. The same investigator (AM) performed all study procedure assessments. Assessments of sensory function were performed before drug administration.

Administration of study drug Ten patients were randomized by a computer generated random list to receive either methylene blue 2mg/kg (10 mg/mL Methyltioninklorid, Apoteket, Umeå, Sweden) or methylene blue 0.02 mg/kg (that served as control), both infused intravenously over 60 minutes. After monitors for electrocardiography, noninvasive arterial blood pressure, and pulse oximetry were attached, a dedicated 20-gauge cannula was inserted into the dorsum of the nondominant hand for administration of the study drugs. The pain was measured at baseline and at 60 min after the start of infusion (NRS scale) and also with a pain diary during the next 24 hours and the following 5 days. ECG, pulse, blood pressure, O2 saturation, were continuously recorded during the infusion. Blood and urine samples were taken before and after the infusion of MB. Neither the subjects of the experiment nor the person examining the patients knew the concentration of MB in the infusion. The infusions of methylene blue in sterile saline (NaCl 0.9%) were prepared by another person who had access to the randomization list but not involved in the monitoring of the patients. The infusions bags were covered in opaque red wrappes and the infusions sets were opaque.

Pain Assessment were performed Before and after MB administration, Evaluation of sensory function was performed using bedside examination according to EFNS guidelines: light touch, pinprick sense, warmth (40°) and cold (25°) temperature stimuli were tested. The contralateral uninjured side served as within-patient control. The patient compared the sensations in both between sides and reported if there was any hypoesthesia, hyperesthesia, allodynia or simply normal sensations to the different stimuli. The pain recordings were determined before and after infusion of MB. Patients kept a diary where they could pick their pain levels on a scale between 0 and 10 (NRS) at every 6 hours after the infusion in the first 24 hours and at 8 hours in the next 5 days.

Peripheral venous blood was drawn from fasting subjects using a 19-gauge needle. Urine was collected into additive-free tubes. Plasma was prepared from blood collected into tubes containing heparin by centrifugation (3500x g for 12 min). Urine and plasma samples were stored at -70◦C until analysis. Blood and urine samples were collected before and after the infusion of MB.

Plasma and urine concentrations of isoprostane 8-iso-PGF2 alpha (an indicator of oxidative injury),Proximity Extension Assay (PEA) has been found to have specificity of detection and analysis of an increased range of target molecules. PEA technology involved in this study a panel of 92 oligonucleotide labeled antibody probe pairs (Proseek assay kit), Non-parametric statistical methods were performed by the author with GraphPad PRISM 5.0 (GraphPad Software, La Jolla, San Diego, CA, www.graphpad.com 5.0). Data are presented as mean and SD with 95% confidence intervals. The level of significance was set at a p value \<.05.