Some drugs which are safe when taken separately are lethal in combination. = Some drugs which are safe when they are taken separately are lethal in combination.其中，which are safe when they are taken separately 是定语从句，Some drugs 是该定语从句所修饰的先行词。在这个定语从句中，when they are taken separately 是时间状语从句，which are safe 是主句，which 和they 指的都是Some drugs 。如果时间状语从句的主语与主句的主语相同，而且从句的谓语动词含有 be 的某种形式，可以把从句的主语和be 的某种形式一起省略（不能只省略其中的一个）。Some drugs which are safe when taken separately are lethal in combination. = Some drugs which are safe when they are taken separately are lethal in combination.的意思是：“有些药物分开服用时是安全的，放在一起服用是致命的。”