Cluster headache (CH) is one of the most painful conditions known. CH affects 1 out 1000 and exists in two well-defined forms: episodic (ECH) and chronic (CCH). Ten to fifteen percent of patients have CCH and have less than three months of pain-free time during a year. Medical treatment for CH is divided into acute abortive treatment for the single attack and a prophylactic treatment. The most commonly used prophylactic, verapamil, decreases attack frequency but does not induce remission and very high doses are needed. Although most therapeutic options ameliorate CH, they may be problematic due to major side effects, unsatisfactory treatment response or availability. Thus, novel treatment options are needed. According to several studies, patients that self-medicate with low doses of the serotonin 2A receptor (5-HT2AR) agonist and psychedelic psilocybin report that this is effective as CH prophylaxis or even to induce remission. So far, no clinical trials to confirm this have been conducted, nor is there any objective measures of brain function in association with psilocybin intake in CH. There is, however, already some evidence from functional magnetic resonance (fMRI) imaging studies suggesting that CH patients have abnormal functional connectivity patterns involving the hypothalamus and distributed brain networks, but the implication of these abnormalities is unknown.

The investigators are conducting a prospective pilot study, evaluating prophylactic effects of psilocybin in CCH using an open-label study design. They're also going to investigate psilocybin's active metabolite psilocin and brain function (fMRI) to identify possible brain mechanisms underlying CCH and treatment response, including the correlation of treatment response with psilocin levels and estimated 5-HT2AR occupancy and the extent to which brain network changes are affected by psilocybin and correlated with treatment response.

Effects of psilocybin on headache frequency, duration and intensity will be assessed in a sample of 20 patients with CCH. Participants will fill out headache logs during the entire study period, in total 10 weeks. Before study inclusion, participants taking prophylactic medication will first go through a 2-week wash-out period to allow for elimination of the medicine. Inclusion is followed by a baseline observation period lasting four weeks, after which patients will first undergo a baseline rs fMRI scanning followed by the first dose of 0.14 mg/kg psilocybin p.o. Blood samples will be collected during the first psilocybin intervention to establish psilocin plasma concentrations, which will be used for estimating receptor occupancy. Participants will then undergo two additional psilocybin administrations spaced by one-week. The last psilocybin dose will be followed by 4 weeks of observation. One week after the last administration of psilocybin, participants will undergo a follow-up MRI scan. Participants will be contacted 3, 6 and 12 months after the last psilocybin dose to gain information about the duration of potential remission periods. All regular acute treatments are permitted during the study period and a systematic record hereof has to be noted in the headache diary. No other prophylactic medication is allowed during the trial and at least a two-week washout period before inclusion is required. Prophylactics are allowed again after the 4 weeks follow-up, with dose and type carefully recorded. Participants will fill out questionnaires during the observation period, in conjunction with psilocybin interventions and at follow-up.