Bullying someone can lead these people to not knowing how to interact with others properly. What it means is that, when people get bullied, they feel like they are judged by others and they also like to mess around with them a lot. And because of these they struggle to do some stuff like interacting with people. Because of the fear that they will get judged, the feeling that everybody is watching your every single movement and knowing that if you do a little thing, they mess with you all the time. This leads to them isolating themselves and becoming a hikikomori (a japanese term that means shut in and isolating themselves). 

This later on makes the individual who has experienced bullying make it harder to communicate with others. How can that impact the person? You may think that since you don't really know about how a person can struggle with communicating, here's the  answer to that question. Well since they have been bullied before they think that if someone says something they might think that’s insulting that person and might not know what to respond with even though that's not what the other person meant, leading to misunderstandings and miscommunications. It can also change in so many ways like your personality, relationships with others etc. An article from NIH stated that ‘Compared to non-involved children, victims were more submissive, had fewer leadership skills, were more withdrawn, more isolated, less cooperative, less sociable, and frequently had no playmates.’(NIH)

Another important thing is that it can also mess up your mental health. People who have been bullied or are still being bullied can cause them to develop harmful ways of coping with that bullying. Some examples of harmful coping ways are; self-harm, alcohol consumption, self-hate, vaping etc. A interview by BMC Public Health founded that ‘Fifteen percent of participating adolescents reported engaging in self-harm during the last year. The risk of self-harm was six times higher for the “bully-victims”, five times higher for the bullied, and three times higher for the bullies, compared to the “neither-bullied nor bullies”. The risk of self-harm in the face of being bullied was significantly greater for girls than boys. Depression, anxiety and parental conflict accounted for some of the association between being bullied and self-harm, and between bully-victims and self-harm.’The reason for doing it is that, the bullies may threaten them like ‘if you tell this to anybody, i swear that i will do stuff that even you can’t think of to you and your parents’ so they have no choice but to obey the bullies. 

Since they can't tell it to anyone they feel trapped with their thoughts and may want to reflect the pain in some way like engaging in something that can possibly be dangerous like self harm. Self harm doesn't necessarily mean cutting oneself; it can be; taking extremely hot showers, hitting yourself, scratching yourself, burning yourself on the kitchen stove, etc. Like said, since they sometimes can't tell it to anyone, they reflect it that way and cause them to develop severe mental illness. 

To prevent this from happening to your loved ones is to notice the signs and help them as much as possible and take action to help them and not just watch it happen if you dont want them do die due to suicide or die in general, dont ignore them realize that they are suffering  and help them.