Complex sentence: He entered medical school in 1990, but left after two years to become Bill Clinton's healthcare lieutenant during the 1992 campaign; he later became a senior adviser in the Department of Health and Human Services after Clinton's inauguration.
Split into simpler sentences: He entered medical school in 1990--leaving after two years to become Bill Clinton's healthcare lieutenant during the 1992 campaign. He later became a senior adviser in the Department of Health and Human Services after Clinton's inauguration.