verb 問題

其實這一句的完整版是：
The quality control in some overseas pharmacies is not strict enough to ensure (that) the drugs are not contamination
所以，你大可以看到，the drugs are not contamination.是一句that－子句，既有that-子句，那麼在that－子句裏，也可以有一個動詞。

希望你明白！

It is because ensure is a special word. After this word, the next verb can be uesd by the base form.