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efficacy
/ˈɛf.ɪ.kə.si/
This word is more formal than "effectiveness." You will most often see it in scientific, medical, or legal contexts when talking about whether a treatment or a law actually works. It is an uncountable noun, meaning you generally do not use it in the plural form (do not say "efficacies" unless referring to different types of effectiveness across various studies).
💬Casual Conversation
🎬Tuesday afternoon, Chloe is in the university library while Ryan is at home gaming.
Chloe Smith
my skin care routine has zero efficacy. i'm literally breaking out everywhere.
Ryan
damn that's crazy. maybe just wash your face?
💡
Chloe uses 'efficacy' to describe the failure of her products in a dramatic, academic-adjacent way typical of a liberal arts student. Ryan responds with his signature oblivious and dismissive phrase 'damn that's crazy', ignoring the nuance of her complaint.