Patients are randomly assigned to 1 of 2 groups to receive cysteine hydrochloride orally twice daily, 2 capsules with breakfast and 2 with lunch. Group 1 receives cysteine hydrochloride in drug ingestion period 1 followed by placebo in period 2. Group 2 receives placebo in period 1 followed by cysteine hydrochloride in period 2. Both groups ingest placebo for 1 week between the periods. Each drug ingestion period lasts 8 weeks.