LSD to Improve Cluster Headache Impact Trial

Inclusion Criteria: * CCH according to the International Classification of Headache Disorders version 3 (ICHD-3) * At screening: stable weekly attack frequency in the 4 weeks prior to screening (assessed retrospectively), averaging at least 8 per week and each week within a 40% window around the average * At randomization: average of at least 8 attacks per week and no absence of attacks on more than two consecutive days during baseline Exclusion Criteria: * Use of excluded concomitant treatment at screening (lithium; other prophylactics if not on a stable dose for less than one month; steroids/GON block within 2 months before screening; sphenopalatinum block, neurostimulation (changed setting within 3 months before screening) or botulinum toxin within 3 months before screening) and during the double-blind phase * Use of LSD(-derivatives) (other than investigational drug), psilocybin, ketamine or cannabis within 3 months prior to screening and throughout the study * Lifetime and/or family history (first degree relatives) of psychotic or bipolar disorder, suicidal intention or attempt * A score of 6 or more on the 'Ervaringenlijst' (PQ-16) to exclude subclinical susceptibility to psychosis * Actual abuse of alcohol and/or recreational drugs * Lifetime history of cardiac valvular disease * History or evidence of cognitive disorder at screening * Positive urine drug screen at screening * Females: Pregnancy, lactation, no acceptable contraceptive use