Nauseated usually means feeling sick or as if you might vomit. If you are talking about your own physical reaction, this is the traditional choice. Example: I felt nauseated after the boat ride.
Nauseous traditionally means causing nausea. It describes something unpleasant that makes another person feel sick. Example: That smell is nauseous.
Here is the easiest way to remember it: if a person feels ill, nauseated is the precise word. If a smell, sight, or experience makes people feel ill, nauseous is the traditional word.
That said, modern English has shifted. Many speakers now use nauseous to mean feeling sick too. Because that usage is common, you will hear sentences like I feel nauseous in everyday conversation.
- Use nauseated for the person who feels sick.
- Use nauseous for the thing that causes the feeling, if you want the traditional distinction.
- In casual speech, nauseous often does both jobs.