NHS staff and terror attack victims and their families have been warned about the risks of using social media in new guidelines issued by NHS England.
The guide cautions that internet trolls may subject victims to "vile and upsetting abuse", as was seen after the Manchester and London terror attacks.
It also warns people to be wary of journalists seeking accounts of events.
The process of retelling a story can make people "relive the worst parts" of horrific events, it warns.
People may say more than they intend to when "vulnerable, upset or angry", it adds.
'Doing something for money'
The NHS England guide also issues specific advice to younger people, cautioning: "People will use your information for their own ends and when you're in the heat of the moment you may say more than you intend or later regret."
Regarding trolls, it tells teenagers: "They can say things like you're only doing something for money or to abuse the system and so on.
"This is incredibly hurtful - which is what the sender intends - and it will upset you, or make you angry and that's never the best time to think about what you tell."
But the guidelines add that social media can also play a positive role in helping victims and families after attacks.
It points out that coverage can help people by appealing for help, getting questions answered, or paying tribute to those who have been killed.
Dan Hett, whose brother Martyn died in the Manchester attack, said he was inundated by media attention.
"I had a couple of bad run-ins with people physically turning up at my house, my parents' house and my place of work," he told the BBC.
"It was incredibly inappropriate. We'd not even confirmed he'd died at this point, so everything was incredibly raw."
Mr Hett welcomes the guidelines: "Overall I'm very much in favour of clear guidance being put in place for people, but to be honest I think some basic decency should really cover most of it."
The guide warns NHS staff against "accidentally revealing something about the incident which is not already in the public domain."
It says: "Often in terror incidents the receiving hospitals are not named in the media straight away, so don't post that you've had a busy shift dealing with victims, as this will confirm something which isn't already known."
It reminds staff to maintain patient confidentiality and cautions them against tweeting rumours.
But Prof Chris Moran, NHS England's clinical lead for trauma, said social media helped doctors respond in the aftermath of the Manchester attack.
"Staff in Manchester were alerted quickly and got to their hospitals before a major incident was even declared," he said.
"This real-time reporting helped NHS staff anticipate injuries, numbers of casualties and what they would be dealing with."
He added: "The flip side to the coin is that misinformation on social media spreads fast and can cause lots of problems, including increased anxiety for patients, families and staff."
This occurred following Manchester when fake images and rumours circulated online.