In bipolar disorder, mania and depression are two different aspects of the same illness. Treatment must address both aspects simultaneously to be effective. 

When a person goes in for treatment he will usually be in either a manic or a depressive phase. When a person is initially in a depressed phase, an antidepressant may be used. The doctor must be careful because antidepressants can cause manic episodes and increase the frequency of mood swings in bipolar patients.
Bipolar depression may be treated with medication for several months, then discontinued to reduce the risk of drug-induced mania.

The medications used to treat the manic phase are called mood stabilizers. Anticonvulsant, antipsychotic or antianxiety medications may be used to help stabilize the patient.

Some bipolar patients do better with combinations of medications rather than just one alone. Combining medications can allow patients to take a lower dose of each. This usually reduces side effects.