Complex sentence: In 1996, dexfenfluramine became the first long-term anti-obesity medication approved in the US; adverse effects observed during clinical trials included dry mouth, diarrhea and drowsiness.
Split into simpler sentences: In 1996, dexfenfluramine became the first drug approved in the U.S as a long-term anti-obesity drug. Observations during clinical trials found adverse effects included dry mouth, diarrhea and drowsiness.