There really is no set definition but one can usually tell if their relationship is poisoning their system. And if they pretend not to....others can usually tell too.

u/Visible_variety wanted to identify how to spot toxic relationships:

What are the telltale signs a couple shouldn't be together?

Here were some of the answers.

When Everytime you meet him or her, he/she's complaining about their SO.

Seriously, I've seen people have kids together but never have anything good to say about each other when the other person is not around.

They only stick together out of fear of being alone.

Sooner be alone and happy than with someone and miserable

There is actually research by John Gottman in this area. He calls them the four horseman.

The traits are contempt, criticism, stonewalling and defensiveness. His research has shown that within just a few minutes of watching a couple, he can determine if their relationship is likely to end in divorce based on if any of these are present.

https://www.gottman.com/blog/the-four-horsemen-recognizing-criticism-contempt-defensiveness-and-stonewalling/

The good news is, they all have antidotes!

CONSTANT fighting. A little arguing back and forth is normal, but when you can hear these idiots screaming their heads off at each other every single goddamn day of their lives, they need to just quit it.

The only one for me is: when they don't like each other.

Literally everything else I've seen couples successfully bypass - extreme differences in personalities, in interests, in politics, in sex drives, in religions, all fine - as long as they like each other and want to be around each other.

If you're dreading going home at the end of the day because your spouse is there, that's the only time I think "Why are you even together, then?"

The only two things they do together is fight and f-ck

I saw one just the other day at a restaurant. He would be talking, and she would be listening and responding. When she was talking, he would pick up his phone and stare at it the whole time.

That relationship isn't going to last.

If you're not comfortable with your SO. I think being comfortable around them is very important. This comes from someone with a lot of anxiety.

Vague, passive aggressive posts on social media that can only be directed at each other.

Venting without using their names.

It's fine to be frustrated with each other but if yall cant be direct with each other it's time to re-evaluate your relationship.

When their relationship doesn't seem as fulfilling without drug use.

When they can't make each other laugh

When their whole personality and behavior changes when their SO is around.

It means that they can't be themselves while being together and results in an unbalanced couple where one (or both) end up unhappy in the long run. It gets tiresome to act like someone you're not.

When every last one of their friends and family thinks so

My best friend married a horrible person. His brother agrees. All his mutual friends agree. My biggest regret is saying nothing at his wedding. But I know him and he'd have just disregarded it and thrown me out

When you ask Reddit for the telltale signs if you should be together.

Marks of violence for starters.

When they don't share core values. It's fine to like different food, entertainment and activities, but if you don't share the big stuff like beliefs, morals, life/family goals, those are big things that can turn into points of contention.

An argument or discussion doesn't always change those things either.

When they host a dinner party with two, and then later on three, couples just to passive aggressively humiliate each other the whole time. And then later on when they've both reached their limit she throws his favorite trophy at the plasma tv, which he bought, and subsequently breaking it.

It doesn't help that prior to this she puts on the music of her young, hot ex-employee that she clearly got ravaged by multiple times.

Pretty much anytime I see a couple have a shared FB account. Usually it seems like either someone cheated, and so they don't fully have the trust of their partner, or someone absolutely wears the pants and the other is a pushover. Not saying that couples can't make those sort of situations work, but...

When one partner sets out huge expectations and outlandish terms expecting the other to change and defer to them. The relationship would be a dictatorship.

I was talking with a new couple, and the girl was telling me how they like none of the same things. Every date they go on, one was sacrificing for the other to do something they liked. At some point in the conversation, I made a joke about sex, and she said, "Oh yeah, there is one thing we both enjoy!"

Sex is nice, but don't let it be your only hobby in common.