Nauseous and nauseated have two different definitions, but recently they have become almost interchangeable. To many people, the following two sentences have exactly the same meaning:
Example 1: I feel nauseated when I am nervous.
Example 2: I feel nauseous when I am nervous.
In both sentences, the speaker is referring to a feeling of queasiness. It is becoming increasingly common to hear the second example above, which uses nauseous in place of nauseated. Actually, these words have two separate meanings. Nauseous is used when referring to something that actually causes a feeling of sickness, while nauseated is used to refer to the actual sensation of sickness, as illustrated in the following sentences:
Example 3 (nauseous-correct usage): The smell of rotten eggs is nauseous.
Example 4 (nauseated-correct usage): The smell of rotten eggs makes me nauseated.
Example 3 illustrates the correct usage of the word nauseous, because it refers to the smell of rotten eggs–something which causes a feeling of sickness. In Example 4, nauseated is used to refer to the feeling of nausea. Just remember that when you are feeling sick, use nauseated, but when you are referring to something that actually causes you to be sick, use nauseous.
Please note that the word nauseating is synonymous with nauseous in that both refer to something that causes nausea. To say, “The smell of rotten eggs is nauseating” would be as correct as saying, “The smell of rotten eggs is nauseous.” The real confusion, however, lies in the difference between nauseous and nauseated.