heroin is cut with carfentanil, fentanyl

The medication used to reverse opioid overdoses could be rendered useless in cases when heroin is cut with carfentanil—a drug that is 10,000 times more potent than morphine and 100 times more powerful than fentanyl.

Google 用於逆轉阿片類藥物過量的藥物在用卡芬太尼（一種比嗎啡有效10,000倍和比芬太尼強100倍的藥物）切割海洛因的情況下可能無效。

cut something with something

to dilute something with something else. They cut the liquor with cold water. Please cut this with some soda. It's toosweet, otherwise.