Lithium is a very strong and potentially dangerous medication used for bipolar. That's why anyone on lithium usually has blood tests done regularly.... This can vary from weekly to 3 monthly, depending on the dosage and the presence of other illnesses.
There are a lot of other medications that are used for the management of bipolar disorder that seem to be quite effective as well.
But, research is still on going to find what would be most effective with the least amount of side effects.