Mark Waldman and Andrew Newberg, M.D. write:

If I were to put you into an fMRI scanner—a huge donut-shaped magnet that can take a video of the neural changes happening in your brain—and flash the word “NO” for less than one second, you’d see a sudden release of dozens of stress-producing hormones and neurotransmitters. These chemicals immediately interrupt the normal functioning of your brain, impairing logic, reason, language processing, and communication.

In fact, just seeing a list of negative words for a few seconds will make a highly anxious or depressed person feel worse, and the more you ruminate on them, the more you can actually damage key structures that regulate your memory, feelings, and emotions.[1] You’ll disrupt your sleep, your appetite, and your ability to experience long-term happiness and satisfaction.

If you vocalize your negativity, or even slightly frown when you say “no,” more stress chemicals will be released, not only in your brain, but in the listener’s brain as well.[2] The listener will experience increased anxiety and irritability, thus undermining cooperation and trust. In fact, just hanging around negative people will make you more prejudiced toward others![3]

via 3. The Most Dangerous Word in the World | Psychology Today.