There are a number of medications that are used that include treatment during and preventitive medicines. The most common are Antidepressants (treatment) and Lithium (treatment and preventitive).
Antidepressant drugs are those that elliviate the symptoms and signs of depression during an episode. When taken, the patient becomes more optimistic and happier than they would be if it wasn't taken.
Although they have side effects like drowsiness and faintness, their overall success is quite good.
The most widely perscribed antidepressant is Prozac. It is also widely controversial, as it is so expensive (a daily dose costs about $2), and it is so new, that the side effects are not widely known. Some scientists suggest that there may be a darker side to this antidepressant that they have not found yet.
However, despite this many doctors are willing to provide it, and the success rate is high.
Lithium is one of the most highly perscirbed, highly effective both preventitive and "during" treatments.
Lithium is a blend of simple mineral salts that is very successful in treating both depression and manic depression.