Meaning: This means that the treatment worked well and had the desired positive result.
Choice of Words: "Treatment" refers to the medical care given to a patient. "Proved" means to demonstrate or establish the truth or validity of something. "Very" means to a great degree or extent. "Effective" means producing a desired or intended result. When you put them together, "Treatment proved very effective" means that the medical care was successful in achieving its intended outcome.