Please recommend me dramas about tough/toxic relationships!

Room No. 9 - thriller

Smile, Dong Hae (2010) - makjang

I'm Sorry, I Love You - the saddest story of all the time

Bad Guy (2010) - perfect execution and the great role of Kim Nam Gil

That Winter, The Wind Blows (2013) - melo

Few recommendations from me:

- Moon Lovers: Scarlet Heart Ryeo - the saddest drama I have seen till now

- The Smile Has Left You Eyes - complicated relationship + sad romance

- Uncontrollably Fond - sad romance

- The Hymn of Death - if you like historical dramas 

- Miracle That We Met - complicated relationships

- Kill It - if you like action and thriller dramas

- A Korean Odyssey - also sad and complicated relationship

And if you haven's seen Goblin, definitely try it  :)

Shitsuren Chocolatier is full of toxic relationships

Well intended love

Behind Your Smile

Hotel King

Definitely Meteor Garden and all of its remakes and sequels. Hana Yori Dango is one of my most memorable. 

Itazaru Na Kiss and all of its remakes and sequels (I realise it isn't quite toxic, but it really takes a toll on the female lead) 

Our Times (again, not quite toxic - but such a difficult time for both of the main characters, finding it so hard to just admit they like each other, some bullying etc) 

He Was Cool (one of my first ever asian movies and always in my heart! This one for me was quite toxic / tough) 

The majority of the dramas and movies I have watched have featured male leads who are quite controlling/jealous/toxic. I wish I could think of more but my mind is blank at the moment. Will come back if I think of any others. So sorry!