Not only does complaining do damage to your body by releasing cortisol and shrinking the part of your brain used in memory and learning, but so does being around people that complain.
Jon Gordon, author of the book The No Complaining Rule, actually likens it to being around second-hand smoke.
The 1996 study from Stanford University took MRIs of brains affected by stress chemicals. Past studies noticed one hormone, in particular, called glucocorticoid caused the dendrites in the neurons of rats and baboons to shrivel after a lengthened exposure. They even noted that "prolonged exposure can kill the neurons or make them vulnerable to destruction during a brain injury or stroke."
The recent MRI scans of humans saw that the most change of those exposed to high levels of glucocorticoid had a reduced hippocampus. According to the study, "the hippocampus is the region of the brain responsible for explicit, declarative memory for knowing a fact like an address or the name of a friend, and knowing that one knows it. Its neurons are rich in glucocorticoid receptors; this is the region where animal studies have shown that stress hormones can damage neurons."
When studying patients that had suffered long term depression, they noticed that the hippocampi were, on average, 15% smaller than those that had not been under stress. Vietnam veterans suffering from PTSD had 26% small hippocampi than those vets that hadn't been suffering.
They stress that this study, alone, is not proof that the glucocorticoids are responsible for the shrinking of the brain, but that other studies also point to this being the case.
Unrelated to the study is Guy Winch, PhD and author of The Squeaky Wheel, who says that "95% of consumers who have a problem with a product don’t complain to the company, but they will tell their tale to eight to 16 people" and that it is this constant misdirection and continually bringing up the problem that does the long term damage. When we are able to focus our complaint to the person or party that needs to hear it, and do it with the aim for a positive resolution, it can actually strengthen relationships.
So remember, complain with a purpose and with the intent to resolve the issue. Don't use it as a tool to vent but as a point of resolution. Break your ego's fear of confrontation and you will be better off for it.