What can loneliness do to your mental health?

It is not good: Loneliness places people at greater risk for depression as well as various health problems. If one is socially isolated and lonely for that reason. It would be helpful to get involved in something that places you among people. If you feel lonely in your current relationships. Talking about this with your significant others and/or a counselor would be helpful

Harmful: Dr. Marcie Berman eloquently answered this. I would add that statistically studies have shown lonliness leads to a shorter lifespan.