Bipolar disorder is a medical condition in which the sufferer goes through periods of extreme mania then depression. There are two different types of Bipolar. What is known as bipolar I is characterized by long periods of mania sometimes lasting as long as a week. Bipolar I can cause the sufferer to exhibit psychotic episodes. In bipolar II depression is the more severe of the mood swings and mania may not be as recognizable.
Meth, on the other hand, is an addictive drug that causes one to appear manic for days on end. Eventually the drug wears off and, because the drug causes a spike in dopamine production, depression ensues. Dopamine is a neurotransmitter that effects the central nervous system. One of it's many functions is to cause the pleasurable feelings that you enjoy in your every day life. The spike makes you feel invulnerable, something that mirrors what bi-polars feel in a manic state. The difference in the two is that the effects of meth last only until the meth wears off. Once the meth wears off the user feels a dramatic drop in dopamine production making it hard to be happy without the drug.
Both cause the sufferer/addict to feel an absence of hunger and loss of the need to sleep. He will feel the continual need to do something or speak, often running on and on without really making any sense. To an outside observer the symptoms are so similar that someone who has dealt with one may confuse it when dealing with the other. However, it's important to note that the symptoms of meth-addiction are temporary.
Because of similarities between symptoms of bi-polar and signs of meth-amphetamine use, it's easy to see where the idea came from that one might cause the other. Although it is true that often people who suffer from debilitating depression (such as is found on the down side of bi-polar) often self-medicate using meth-amphetamines or cocaine, there is no evidence that bi-polar is caused by this.
Prolonged use of meth-amphetamines can cause a lifelong battle with depression however. This is the result of wearing out the central nervous system's ability to produce dopamine. Bi-polar disorder is a continuous and ongoing problem which is solved by leveling the amount of chemicals the brain produces. This can be done by certain medications developed for this purpose. If you feel that you or a loved one may be suffering from bi-polar you need to consult a physician immediately.
*Sources: Personal experience, webmd, PBS.org, lifeloveand bi-polar.com, livestrong.com, bipolarworld.net