Complex sentence: Carbamazepine became a popular treatment option for bipolar in the late 1980s and early 1990s, but was displaced by sodium valproate in the 1990s, which has become a commonly prescribed treatment, and is effective in treating manic episodes.
Split into simpler sentences: Carbamazepine became a popular treatment option for bipolar in the late 1980s and early 1990s, but was displaced by sodium valproate in the 1990s. Since then, valproate has become a commonly prescribed treatment, and is effective in treating manic episodes.