A. wear outB. put downC. put offD. wear off

【答案】D

【解析】

考查动词短语辨析。句意：这种药的药效在几个小时内就会自然消失。A. wear out 磨损；耗尽；B. put down记下；C. put off推迟；D. wear off逐渐消失。根据句意可知表示药效逐渐消失。故选D。