Both "nauseated" and "nauseous" derive from "nausea," the feeling of stomach discomfort that might lead to vomiting. You use "nauseated" as a past tense verb: "I am nauseated." You use "nauseous" as an adjective meaning causing nausea: "The smell of rotten meat is nauseous." In other words "nauseous" refers to the cause of nausea, while being "nauseated" is to suffer from it.

"Nauseous" Examples

"Nauseous" as an adjective describes the source of what makes you sick: "The nauseous feeling arose from the nauseous smell." Reference.com notes that it can also follow a verb of appearance, change or state of being: "She looks nauseous" or "He became nauseous." As an adjective, it can modify a noun that causes nausea: "We climbed to nauseous heights."

"Nauseated" Examples

"Nauseated" is a verb that you can use with an object -- "His appearance nauseated me" -- or without one -- "I am nauseated." It's important to remember that "nauseated" is still a verb, even with a helping verb. "She feels nauseated" is a verb phrase.