Doctors expect that the medicine will be effective. 主动句

It is expected that the medicine will be effective. 被动句

The medicine is expected to be effective. 提升主语结构

当比较分句为以上主动句时，则为：

The medicine is more effective than doctors expect (it will be).

当比较分句为以上被动句时，则为：

The medicine is more effective than (it) is expected (that it will be).

当比较分句为以上提升主语结构时，则为：

The medicine is more effeective than (it) is expected (to be).

括号内为省略的部分，可以补全。

than为连词。在非正式语体中有时than被用作介词，受到部分学究的质疑。下句为非正式语体：

The medicine is more effective than what is expected.

what为融合型关系代词，= the thing which，引导的从句相当于一个名词词组，即than后接的是名词词组，无法扩充为一个分句。