单选题 The doctor informed his patient that the drug was very ______ and can have unpleasant sideeffects.

【正确答案】 A

【答案解析】[解析] 各项的意思是：potent“(药等)效力大的，强有力的”；efficient“有能力的；效率高的”；intricate“错综复杂的”；fragile“易碎的；虚弱的”。根据句意判断，答案是A。