It's very dangerous for you to use a kind of medicine TO which you are allergic.
为什么用to啊☹️
固定搭配 be allergic to...,意思是“对...过敏”。
根据定语从句的语法:(to which) you are allergic = you are allergic (to which) = you are allergic (to a kind of medicine),即关系代词 which 指代先行词 a kind of medicine,并在该定语从句中充当 be allergic to 中介词 to 的宾语。