A lot of times in media and in Real Life there's a friend (or a group of friends), a very good friend whom you may have known since you were kids. But there's one small flaw within the friendship. The friend in question always gets you into serious trouble. It's usually stuff like trying to get you to break curfew, violate your parole, drink underage, do drugs, commit acts of theft, and cause public disturbance. The toxic friend will guilt-trip you for not going along with what they want and accusing you of "changing". They'll say things like, "Man, you've changed. You used to be cool," or, "Come on, it'll be fun!" And of course, all of the above will almost certainly lead to serious consequences. This is almost the 2000s variant of The Aggressive Drug Dealer.
More often than not these characters are usually plot devices for the protagonist to get into a horrible situation for the purposes of An Aesop. Under normal circumstances, the protagonist in question is usually a rational thinker when not around these types of friends. But when they hang around them, they become ridiculously impressionable and naive and easily misled. The friend could be generally well-intentioned and benevolent, but still not a good friend in the greater scheme of things. Also expect these friends to be from the Wrong Side of the Tracks, while the protagonist usually has a squeaky-clean square background (though not always). If they're malicious, they may try to cut off their new friend from old ones because, "They Were Holding You Back." If the friend is female, expect her to be troubled and possibly promiscuous.
The toxic friend could also possibly be a passive-aggressive Et Tu, Brute? and saboteur, like a crabs-in-a-barrel kind of thing. Can be a comedic trope but more often than not it has darker implications. It's also possible for the Toxic Friend to be a significant other as well. Oh, and expect the Toxic Friend to have a very low mortality rate. In a darker sense, serial killers can be Toxic as well by turning their friends, family, and lovers into accomplices.
Sometimes, the friend they’re influencing will be aware of their flaws, but Turn the Other Cheek out of the belief that the toxic friend has Hidden Depths that makes their bad influences worth standing by them — until the toxic friend goes much too far with their misdeeds, making the good friend realize they is no Hidden Heart of Gold under their toxicity, leading to a Broken Pedestal and Post-Support Regret.
Sensei for Scoundrels is this trope applied to the student/mentor relationship. See also Peer Pressure Makes You Evil. May also overlap with Femme Fatale.
See also The Corrupter, who has a different kind of bad influence. Contrast Staging an Intervention. The Morality Pet and Morality Chain could be considered the inverse of this trope; their influence could serve as a counter to the Toxic Friend Influence and in fact, influence their troublemaking friend to become a better person. Positive Friend Influence is the polar opposite of this trope.
