Specific aims: The null hypothesis of the study implicates that l-cysteine is no better than placebo in migraine prophylaxis during the intervention period of 3 months. Rejection or not of the null hypothesis is based on comparison of the two arms for two primary study endpoints and (to lesser extent) for a series of secondary endpoints. The two primary study endpoints (efficacy measures) are: a) Number of migraine attacks (NMA) per evaluation interval (1 month), and ii) Number of migraine days (NMD) per evaluation period. Potentially useful secondary endpoints include: i) Intensity of headache (4-tier nominal scale); ii) Attack duration in hours (potentially biased by treatment); iii) Drug consumption for symptomatic or acute treatment (NMDs treated with abortive agents and the number of drug administrations for acute therapy); iv) Patients' preferences and satisfaction; v) Responder rate (proportion of study subjects with >50% improvement in NMA or NMD, as compared to baseline values).