Frequently quoted at people who are struggling. The second line is the half that matters.
Quick answer
Gladness works on the body like a remedy. And a crushed spirit does real physical damage, which the verse states as a fact rather than a fault.
Read both lines
The Hebrew word in the first line is rare, which is why the NKJV adds a footnote offering or makes medicine even better, and the honest position is that translators are working with a difficult term. What is not in doubt is the second line. A broken spirit dries the bones, an image of something essential going out of a person, and the book uses drying and rottenness of the bones elsewhere for what degrades a person structurally. The verse's claim is that interior states have physical weight, which anyone who has been through a hard season already knows in their body.
What it does not mean
This is not an instruction to cheer up, and it is not a diagnosis of anyone who cannot. The verse assigns no blame to the person whose spirit is broken; it reports the cost. Nor does it teach that a positive attitude cures illness or that a sick person lacked one. Depression, grief, and chronic pain are real and can need real care, and no proverb about a merry heart overrules that. What the verse establishes is that these things are not merely mental, which is a reason to take a struggling person's exhaustion seriously rather than a reason to tell them to feel differently.
Related: Proverbs 15:13 · Proverbs 12:25 · Proverbs 18:14 · Proverbs 14:30
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.