Study design: A double-blind, randomized placebo-controlled multi-centre trial comparing Acetium capsules (100mg L-cysteine, twice a day) and placebo in prevention of headache attacks during a 1-month trial period. In brief, a cohort of 60 (up to100) voluntary subjects (women and men, with periodic or chronic cluster headache) are invited through the Finnish Migraine Association (FMA). To be eligible for the study, the subjects should: i) have the frequency of typical attacks up to 5 per day; ii) age between 18 and 65 years; and iii) have a minimum of co-morbidity. There are no restrictions on duration since the diagnosis, or the age at the onset of cluster headache. They should discontinue other prophylactic medication prior to study entry. Before enrolment, all subjects are requested to sign a written consent. The study protocol will be subjected for approval by the Committee on Medical Research Ethics (HUS).