Clinical and Mechanistic Effects of Psilocybin in Alcohol Addicted Patients

Inclusion Criteria: * Informed Consent as documented by signature * Right-handedness according to Oldfield (1971) perfomed during the telephone screening, laterality index ≥ 0.2 * DSM-IV-diagnosis of alcohol use disorder (based on clinical assessment and confirmed by the SCID Interview) * Having undergone withdrawal treatment from alcohol use or have stopped consuming alcohol within 6 weeks prior to enrolment in the study * Drug free from any psychotropic and serotonergic medication for at least five days before administration of the study drug or placebo * No alcohol use between withdrawal treatment and administration of study drug or placebo * Good physical health with no unstable medical conditions, as determined by medical history, physical examination, routine blood labs, electrocardiogram, urine analysis, and urine toxicology * Normal level of language comprehension (German or Swiss-German) * Willing to refrain from drinking caffeinated drinks during the testing days and from consuming psychoactive substances after enrolling in the study and for the remainder of the study * Women of childbearing potential must be using an effective, established method of contraception for the entire study duration, such as oral, injectable, or implantable contraceptives, or intrauterine contraceptive devices. Note: female participants who are surgically sterilised / hysterectomised or post-menopausal for longer than 2 years are not considered as being of child bearing potential. * Have a family member or friend who can pick them up and stay with them overnight after the psilocybin administration sessions (driving is forbidden at drug treatment days) * No further medication is allowed until visit 6, except for emergencies Exclusion Criteria: * Allergy, hypersensitivity, or other adverse reaction to previous use of psilocybin or other hallucinogens * uncorrected Hypertension (assessed at screening day: higher than 139 systolic and 89 diastolic) * Women who are pregnant or breast feeding * Intention to become pregnant during the course of the study, * Lack of safe contraception, defined as: Female participants of childbearing potential, not using and not willing to continue using a medically reliable method of contraception for the entire study duration, such as oral, injectable, or implantable contraceptives, or intrauterine contraceptive devices, or who are not using any other method considered sufficiently reliable by the investigator in individual cases (Female participants who are surgically sterilised / hysterectomised or post-menopausal for longer than 2 years are not considered as being of child bearing potential) * Known or suspected non-compliance * Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant, * Previous enrolment into the current study * Enrolment of the investigator, his/her family members, employees and other dependent persons * Lifetime history of bipolar disorder (I, II, not otherwise specified) Lifetime history of schizophrenia, schizoaffective disorder, or psychosis not otherwise specified * History of DSM-IV drug dependence other than alcohol (except for caffeine or nicotine) within two months prior to enrolment * Comorbid Axis I anxiety and depression disorders diagnoses as well as post-traumatic stress disorder will be permitted if they do not require current treatment * Family history of schizophrenia or schizoaffective disorder, or bipolar disorder type 1 (first or second degree relatives) * Violent behaviour within last 2 years or history of suicidal behaviour * Lifetime history of hallucinogen use on more than 10 occasions within last 10 years * Getting psychotherapeutic or psychological treatment from third parties during the study is forbidden * Abnormal electrocardiogram * Any unstable illness as determined by history or laboratory tests * BMI \<17 or \>30 * Uncorrected hypo- or hyperthyroidism * Contraindications to magnetic resonance imaging (MRI safety form) * During the study, new use or dose changes of already existing concomitant medication without prior informing the investigators is forbidden * High risk of adverse emotional or behavioural reaction based on investigator's clinical evaluation (e.g., evidence of serious personality disorder, antisocial behaviour, serious current stressors, lack of meaningful social support) * Participation in another study with investigational drug within the 30 days preceding and during the present study * Taking medications that are known to modulate uridine diphosphate glucuronosyltransferase enzyme * Inhibitors of UGT1A9 and 1A10 should be discontinued at least five half-lives prior to the administration of psilocybin * Monoamine oxidase and aldehyde or alcohol dehydrogenase inhibitors should be discontinued at least 5 half-lives prior to the dose of psilocybin