____________________ is a useful substitute for morphine and heroin, because it satisfies the craving without disrupting behavior as greatly as morphine and heroin do.

Substitute

A person or thing that becomes the replacement or alternative for someone or something else.

Morphine

A potent opiate analgesic drug derived from the opium poppy, used primarily for the relief of severe pain.

Heroin

An illegal opioid drug derived from morphine, known for its highly addictive properties and potential for abuse.