In the short-term acute phase, sertraline will be initiated at 25 mg, increased to 50 mg on day 3, and maintained until day 14. If patients show an early response (i.e. ≧ 20% improvement in the MADRS total score from baseline), sertraline will be continued and titrated at 50 - 100 mg based on clinical judgment. On the other hand, if patients show no early response, they will be randomly divided into two groups. In one group, sertraline will be continued and titrated at 50 - 100 mg, whereas in the other group sertraline will be switched to paroxetine. Paroxetine will be started at 10 mg on days 15 and 16, increased to 20 mg on day 17, and further increased weekly by 10 mg from week 4 (i.e. day 22), while sertraline will be tapered by 25 mg each on days 15 and 16. In case patients are intolerant to adverse events, or they achieve remission (i.e. the MADRS total score ≦ 8), increasing the dose will be terminated. Lorazepam, lormetazepam, and mosapride will be allowed on a p.r.n. basis.