The Impact of Psilocybin on Pain in Fibromyalgia Patients

Inclusion Criteria: * Age between 18 and 65 years * Normal weight, body mass index (weight/height2) between 18 and 28 kg/m2 * Fulfilment of the American College of Rheumatology criteria for FM diagnosis (43) * A minimum Numeric Rating Scale (numeric rating scale) pain score of 5 out of 10 * Proficient knowledge of the Dutch or English language * Written Informed Consent * Understanding the procedures and the risks associated with the study * No regular use of psychotropic medication such as opiates, antidepressants, muscle relaxants, anticonvulsants, sleep aids, benzodiazepines. Non pharmacological regimens will be allowed along 1 rescue therapy such as acetaminophen ≤4,000 mg/day, ibuprofen ≤1,200 mg/day, naproxen ≤660 mg/day, or ketoprofen ≤75 mg/day. Use of paracetamol (PCM) and non-steroidal anti-inflammatory drugs (NSAIDS) will be allowed and monitored. * Willingness to refrain from taking psychoactive substances during the study. * Willingness to drink only alcohol-free liquids and no coffee, black or green tea, or energy drinks after midnight of the evening before the study session, as well as during the study days * Willingness not to drive a traffic vehicle or to operate machines within 24 h after substance administration Exclusion Criteria: * Presence of any other painful condition such as inflammatory rheumatic diseases, migraines or headaches and of other chronic or acute medical conditions * Presence or history of any other psychiatric condition such as primary major depressive disorder, anxiety disorders or substance use disorder as determined by the medical questionnaire, drug questionnaire and medical examination * Previous experience of serious side effects to psychedelic drugs (anxiety or panic attacks) * Tobacco smoking (\>20 per day) * Excessive drinking (\>20 alcoholic consumptions per week) * Psychotic disorder in first-degree relatives * Pregnancy or lactation * Hypertension (diastolic \> 90 mmHg; systolic \> 140 mmHg) * History of cardiac dysfunctions (arrhythmia, ischemic heart disease…) * For women: no use of a reliable contraceptive