Treatment with LDN-placebo for 12 weeks, including af titration phase of 4 weeks. Initial dosage of 1.5 mg, dose is increased by 1.5 mg every 7th day to 6 mg at week 4. Dosing is increased based on safety and tolerability and delayed increments are allowed. The participants are maintained on the highest tolerable dose for the last 8 weeks of the treatment period.

Measured by the Fibromyalgia Impact Questionnaire Revised (FIQR) total score. The FIQR asks patients to rate how difficult it is to perform a list of 9 common activities over the previous 7 days on an 11 point scale, ranging from 0-10 (0 = "no difficulty" and 10 = "very difficult"). The FIQR then asks patients to indicate how often their fibromyalgia impacts their quality of life over the last 7 days on an 11 point scale, ranging from 1-10 (0 = "never" and 10 = "always"). Finally, the FIQR asks patients to assess the severity of 10 different symptoms on an 11 point scale, ranging from 0-10 (0 = "no symptoms" and 10 = "extreme symptoms"). These three sub-scales are summed to represent an overall FIQR score. A lower score indicates lower severity.

Assessed by the Widespread Pain Index (WPI) from The American College of Rheumatology 2016 revised criteria for fibromyalgia (ACR-2016). The body is divided into 19 body parts and number of pain full body parts during the last 7 days is counted, giving rise to the WPI. A lower score indicates lower spreading of pain.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of pain" question, asking the participants to rate the average level of pain during the last 7 days on a 11 point rating scale, ranging from 0-10 (0 = "no pain" and 10 = "unbearable pain"). A lower score indicates lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of tenderness to touch" question, asking the participants to rate the average level of tenderness to touch during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "no tenderness" and 10 = "very tender"). A lower score indicates a lower severity.

AND

Assessed by measurement of pressure pain threshold, using a handheld algometer. Points measured: Right Quadriceps 15 cm proximal of apex patella and left Trapezius 10 cm from acromion. Each point is measured 3 times. Pressure pain threshold is measured in kilopascal (kPa). A higher value indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of energy" question, asking the participants to rate the average level of energy during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "lots of energy" and 10 = "no energy"). A lower score indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "quality of sleep" question asking the participants to rate the average quality of sleep during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "awoke well rested" and 10 = "awoke very tired"). A lower score indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of depression" question, asking the participants to rate the average level of depression during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "no depression" and 10 = "very depressed"). A lower score indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of anxiety" question, asking the participants to rate the average level of anxiety during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "not anxious" and 10 = "very anxious"). A lower score indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of memory problems" question, asking the participants to rate the average level of memory problems during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "good memory" and 10 = "very poor memory"). A lower score indicates a lower severity.

Assessed by the Fibromyalgia Impact Questionnaire Revised "level of stiffness" question, asking the participants to rate the average level of stiffness during the last 7 days on an 11 point rating scale, ranging from 0-10 (0 = "no stiffness" and 10 = "severe stiffness"). A lower score indicates a lower severity.

Assessed by the physical function domain of Fibromyalgia Impact Questionnaire Revised; asking participants to rate how difficult it is to perform a list of 9 common activities over the previous 7 days on an 11 point scale, ranging from 0-10 (0 = "no difficulty" and 10 = "very difficult"). The score is characterized by the sum of the 9 scores (0-90). A lower score indicates better function.

Measured by the European Quality of Life 5 Dimensions (EQ-5D) mobility domain; asking the participants to rate their mobility on a 1-5 Verbal Rating Scale, a score of 1 indicating no problems, and a score of 5 indicating extreme problems. A lower score indicates a lower severity.

Measured by the European Quality of Life 5 Dimensions (EQ-5D) self care domain; asking the participants to rate their ability for self care on a 1-5 Verbal Rating Scale, a score of 1 indicating no problems, and a score of 5 indicating extreme problems. A lower score indicates a lower severity.

Measured by the European Quality of Life 5 Dimensions (EQ-5D) usual activities domain; asking the participants to rate their ability to perform usual activities on a 1-5 Verbal Rating Scale, a score of 1 indicating no problems, and a score of 5 indicating extreme problems. A lower score indicates a lower severity.

Measured by the European Quality of Life 5 Dimensions (EQ-5D) pain/discomfort domain; asking the participants to rate their level of pain or discomfort on a 1-5 Verbal Rating Scale, a score of 1 indicating no problems, and a score of 5 indicating extreme problems. A lower score indicates a lower severity.

Measured by the European Quality of Life 5 Dimensions (EQ-5D) anxiety/depression domain; asking the participants to rate their level of anxiety or depression on a 1-5 Verbal Rating Scale, a score of 1 indicating no problems, and a score of 5 indicating extreme problems. A lower score indicates a lower severity.

Measured by European Quality of Life Visual Analogue Scale (EQ-VAS), which asks the participant to rate their overall health 'today' on a 0-100 VAS. A higher score indicates a better quality of life.

A responder is defined as a subject who reports a more than 15% improvement of the primary outcome (average pain during the last 7 days) after 12 weeks of treatment with LDN or LDN-placebo. Number of responders are calculated for both the LDN group and the LDN-placebo group.

A responder is defined as a subject who reports a more than 30% improvement of the primary outcome (average pain during the last 7 days) after 12 weeks of treatment with LDN or LDN-placebo. Number of responders are calculated for both the LDN group and the LDN-placebo group.

A responder is defined as a subject who reports a more than 50% improvement of the primary outcome (average pain during the last 7 days) after 12 weeks of treatment with LDN or LDN-placebo. Number of responders are calculated for both the LDN group and the LDN-placebo group.

Measured by an isometric muscle exhaustion test of the deltoid muscle. Measured by asking the participant to complete an isometric muscle exhaustion task by maintaining 90° shoulder abduction (dominant arm) for as long as possible with the elbow extended and the hand pronated (hand facing downwards). Task failure (test position can no longer be maintained) defines the test duration. The participants are instructed to maintain the position until they feel maximal muscle exhaustion (corresponding to a score of 10 on a 0-10 scale with 0 being "no muscle fatigue" and 10 being 'maximal muscle fatigue/exhaustion'). Surface electromyography (EMG) will be recorded from the anterior, middle and posterior deltoid muscle at 3000 Hz during the entire test. A longer duration of test is indicative of lower levels of exhaustion.

Measured by the 30-second chair stand test. The test starts with the subject seated on a chair with her back touching the backrest. Subjects rise to a full stand with the back straight and then sit down again to regain this cycle as many times as they can within 30 seconds. A higher number of cycles is indicative of better physical fitness.

Assessments are conducted as recommended by the 'International Association for the Study of Pain'. Computer-controlled cuff algometry is performed on lower legs in all patients to assess pressure pain threshold and pressure pain tolerance. Pressure pain threshold and pressure pain tolerance are measured in kilopascal (kPa). A higher value is indicative of lower severity.

Measured by Conditioned pain modulation (CPM). Assessments are conducted as recommended by the 'International Association for the Study of Pain'. Computer-controlled cuff algometry is performed on lower legs to assess increase in pressure pain threshold during cuff pain conditioning on the contralateral leg. Pressure pain threshold and pressure pain tolerance are measured in kilopascal (kPa). A higher value is indicative of lower severity.

Measured by Temporal Summation of Pain (TSP). Assessments are conducted as recommended by the 'International Association for the Study of Pain'. Computer-controlled cuff algometry is performed on lower legs to assess increase in pain scores to ten repeated stimulations. Pain is rated on a 0-100 Visual Analogue Scale. A lower value indicates a lower severity.