Proverbs 12:18:

There is that speaketh like the piercings of a sword: but the tongue of the wise is health.

Notes on Proverbs 12:18:

Your tongue can bring healing or sickness to others.  It is so important to watch your words.  You could discourage someone and make him more susceptible to sickness, or cheer the person up and make him more likely to get healed.  Our tongues should speak edifying things that will bring health to others.  (Note that telling people that their sickness is the will of God or some other such nonsense is not wise and it will not bring health to anyone!)

See also:

Say What