I have Bipolar disorder (bipolar 1). I began smoking about a month ago and found it to be really relaxing and helped me with sleep. But then I got some more weed that was different, and though enjoyable, was not as relaxing. I started learning about the different strains a little bit and realized that most people just talk about "marijuana" as if it is all the same stuff. But there are many different types. There are two basic types, Sativas and Indicas, and many hybrids of the two. A Sativa is radically different from an Indica.
For myself, being prone to mania, I would probably tend to do better with the more relaxing Indica-dominant strains. The Sativas, from what I understand, are more stimulating, cerebral, more of a high than a stone, and head-trippy, almost hallucinogenic if potent enough, and that would not be good for me most of the time. The problem is, unless you grow it yourself or know the grower, it is pretty hard to get clear info about the weed you are smoking.