Specific aims: The null hypothesis of the study implicates that l-cysteine is no better than placebo in prophylaxis of cluster headache during the 1-month intervention period. Rejection or not of the null hypothesis is based on comparison of the two arms for the primary study endpoint and (to lesser extent) for a series of secondary endpoints. The primary study endpoint (efficacy measure) is the number of attacks per week. Potentially useful secondary endpoints include: i) intensity of headache (4-tier nominal scale); ii) drug consumption for symptomatic or acute treatment; and iii) patients' preferences and satisfaction.