The study lasts for 64 weeks (about 16 months) and is divided into 2 parts, each part lasting for 32 weeks (8 months). During the first part study medication (magnesium or placebo) will be taken twice daily for 24 weeks (about 6 months), followed by no study medication for 8 weeks. These 8 weeks are called a washout period, during which time the effects of the study medication are 'washed-out' of the body. After the washout 8 weeks, patients will enter the second part of the study. During this second part, the study medication (magnesium or placebo) will again be taken twice daily for 24 weeks. During the second part of the study participants will receive whichever study medication that was not taken during the first part of the study. Following the second 24 weeks, there will be an observation period of 8 weeks (during which time no study medication is taken). The study is completed at the end of the 8-week observation period. Participants will take magnesium for 24 weeks and placebo for 24 weeks. The decision whether participants will receive magnesium or placebo first will be made randomly by a computer. This is a double-blind study which means that neither the participant nor the study doctor will know which study medication is taking during each part of the study. However, the study may be unblinded for safety issues if necessary.