Doctors also believe that bipolar disorder is partly genetic.
It can affect several people in the same family.
But it is not usually caused by a single gene.
And finally the environment a person grows up in is important.
Doctors think that bipolar disorder may be partly caused by emotional pain - especially emotional pain that people experience as children.
The sickness usually develops when a child becomes a young adult.
It may start with a stressful event like a divorce or a job loss.
Or it may start with the use of illegal drugs.
There is no cure for bipolar disorder.
But there are several kinds of medicine that can control it.
The traditional medicine for bipolar disorder is lithium.
Lithium helps to control both the high and the low emotions.
But many people do not want to take lithium.
It does help people with their sickness.
But it causes side effects.
The side effects trouble some people.
For example, lithium can make people experience very few emotions.
Some people do not even feel like the same person.
Also people must take lithium for a long time to keep their emotions calm.
If they stop taking it they may become sick.
Although there are some problems with lithium, it helps many people to deal with their sickness.