I have been waiting to see where this went before I chimed in because I often wonder what people know about bipolar disorder. When I was younger I was diagnosed as bipolar and went through countless medications and counselors only to find out that I only had two choices. One was to take lithium all the time and be void of all emotions. The other was to fight the disease and learn to control impulses I cant even begin to describe. Needless to say, I took the hard path and have fought like crazy to get where I am at. Now, I study mental disorders on my path towards becoming a psychologist. Lurkette and others are right when they say there is no single cause for manic depression. There is a genetic and an environmental component to bipolar disorder but research hasn't set any clear boundaries yet. If you want a decent read on manic deppression you can check out this link (it is way too long to post)
http://www.nimh.nih.gov/publicat/bipolarresfact.cfm
I do want to clear one thing up. Drugs do not cause manic deppression. People who have manic deppression, on the other hand, are more likely to use drugs than those who do not. For me, drugs (pot, painkillers, alcohol) were very helpful because they brought me down from manic episodes that always got me in trouble. Pot was probably the most helpful because it slowed down the thoughts that spun around in my head and let me sleep. As I started to understand my mind better, I was able to step away from the drugs and deal with my mania intelligently. I cant stress enough how hard it is/was to get where I am. So, please take it easy on people that have this disorder.
Now that I have dropped all my excess baggage on you guys I have one tip for dealing with someone in a manic episode: DO NOT LET THEM DRIVE.