Sometimes, people say horrible things on Twitter — but how do you know if a nasty tweet crosses the line into legally-actionable defamation?

"Don't be a jerk," is the legal advice of Owen Bourns, a civil litigation lawyer. He spoke to CBC's Information Morning on Monday.

More specifically, don't tweet something that's not true and could be seen as defamatory. 

'Fair comment pertains to issues of public interest, which is very different from what interests the public.' - Owen Bourns

"The [test] is whether the comment would lower the reputation of the subject matter of the comment in the mind of a reasonable person," he says.  

That could apply if you tweet a series of messages where one taken out of context could appear defamatory.

Merely linking to defamatory content won't likely get you in trouble, as linking has been found not to be defamatory in Canadian cases.

But retweeting a defamatory tweet poses a bigger risk.

"You're taking the entire substance of the offensive comment and you're sending it out there to your followers," Bourns says. "If the original tweeter only had 10 followers and you have a couple hundred thousand, you really are the cause of most of the harm."

In a recent high-profile case, TSN apologized for airing a fan's tweet about Dion Phaneuf of the Toronto Maple Leafs. Accused Twitter users, like all Canadians accused of defaming someone, have legal defences.

"The two most common would be truth, which is a full-stop defence, and fair comment," Bourns says. "Fair comment pertains to issues of public interest, which is very different from what interests the public."

If you think you've been defamed on twitter, Bourns suggests finding out who tweeted it, taking screen shots, and getting a lawyer. Bear in mind that may give the comments a much bigger audience through media coverage.

Also, as Twitter is a U.S. company, pursuing legal action from Canada could be a long, costly affair. The U.S. uses very different defamation laws than Canada, meaning what's defamatory here might be fine there.