例264　Dependence is marked first by an increased tolerance, with more and more of the substance required to produce the desired effect, and then by the appearance of unpleasant withdrawal symptoms when the substance is discontinued.

【结构分析】主语是Dependence，动词is marked by，后面还有一个by，省略了marked，这两个动词是并列关系，第一个宾语是an increased tolerance, with more and more of the substance required to produce the desired effect是个伴随的介词短语修饰前面一句话，第二个宾语the appearance of unpleasant withdrawal symptoms，后面有状语从句修饰第二个句子。

【亮点回放】此句话的两个并列的宾语，相当于两个并列句子，第一个并列句子，后面加上with结构，做状语，修饰第一个句子，第二个并列句子，后面加上状语从句when，修饰第二个句子。

【核心词汇】

dependence　药物的依赖性

marked by　表现为

increased tolerance　不断增加的容忍力

desired effect　理想的效果

unpleasant withdrawal symptoms　不舒服的停药的症状

discontinued　终止

【参考译文】药物的依赖性最初表现为耐受力增强，用量越来越大才能达到预期效果，一旦停用就会产生不舒服的症状。