一道语法问题

   Unlike spices that are used in cooking，in using spices for medicine，they are taken in large quantities in order to treat particular maladies.

请问划线部分哪错了 谢谢  ！！

比較結構的原則之一是比較的對象要有可比行,概念相等,形式平行,

原句想要比較的對象是spices used in cooking VS spices used for medicine

而句子把後者變成一個介詞短語,比較對象成了spices used in cooking VS they

但是they指前者還是後者呢?指代不清.

我把句子改寫一遍:Unlike spices (that are) used in cooking, spices used for medicine are taken in large quantities in order to threat particular maladies.

如果有錯請指正!