The Vietnamese word "biến chứng" is primarily a noun that means "complication" or "side-effect." It is often used in medical contexts to describe additional problems that arise as a result of an existing condition or illness.
As a noun: You can use "biến chứng" to talk about complications that occur due to diseases or medical treatments.
As a verb: "Biến chứng" can also be used as a verb, meaning to produce a complication or side-effect.