A transitive verb meaning to aggravate or complicate something — to make a condition (illness, relationship, issue) worse or more entrenched.
風邪をこじらせて入院してしまった。
He aggravated his cold and ended up hospitalized.
問題を放置すると関係がこじらせることがある。
If issues are left unaddressed, relationships can become strained.
小さな誤解を放っておいたら、事がこじらせてしまった。
I left a small misunderstanding alone, and it ended up getting worse.