"Words are, of course, the most powerful drug used by mankind."

Kipling is basically saying that words work on us the way drugs do: they can heal, harm, inspire, or numb us, often more than we realize. Think about a time someone told you, “I’m proud of you” or “I believe in you”, how that one moment carried you for days. Or the opposite: a careless comment that still echoes in your head years later.
We sometimes treat words like they’re cheap and disposable, but they change moods, relationships, even the direction of a life. A heartfelt apology, a hard truth said gently, a simple “How are you, really?”, those are tiny doses of something powerful. Kipling’s reminder is that we’re all pharmacists of language, handing out medicine every day. It’s worth choosing our prescriptions carefully.