Stupefacient is a word used to describe something that induces a state of stupor or lethargy. Some synonyms for stupefacient include sedative, narcotic, anesthetic, hypnotic, soporific, tranquilizer, and narcotic. These words all refer to substances or actions that cause drowsiness, sleepiness, or a lack of consciousness. In the medical field, these synonyms are often used to describe drugs or medications used to relieve pain, relax muscles, or induce sleep. In casual conversation, they may be used to describe something that is boring or monotonous enough to make someone feel lethargic or dull. Regardless of the context, the synonyms for stupefacient all refer to something that has a calming, soothing effect on the mind and body.