If you agree to participate in this study, you will be taking glycine or a placebo for 12 weeks - in two 6-week periods. These two 6-week periods are separated by a 2-week "wash-out" period where you take no study medication. During one of the 6-week periods, you will be taking glycine. During the other 6-week period, you will be taking placebo. Neither you nor the study staff will know in which order you receive the glycine or placebo. In case of an emergency, we can quickly find out which medication you have been taking.