Marijuana is considered a hallucinogen by the US drug laws
Alcohol is considered a disassociative
Cocaine, nicotene and caffiene are considered amphetamines.

They're all drugs. The anti-depressants you get from your doctor are also drugs. All drugs have side-effects which differ from person to person. What cures one person may not cure another.
Marijuana has shown many medical uses in easing the pain of glaucoma and many other chronic diseases. It's also shown to help a lot in reducing anxiety, but whether its truly shown to help with bi-polar disorder is up to your own research. It may make a bi-polar person feel more relaxed, but I dont know that it helps in balancing the off-set chemicals in your brain that are affected by bi-polar disorder.

-Icey-