If you're nauseated you're about to throw up, if you're nauseous, you're a toxic funk and you're going to make someone else puke. These words are used interchangeably so often that it makes word nerds feel nauseated!

Nauseated is how you feel after eating funnel cake and riding the tilt-a-whirl, when you're two months pregnant, or any other time you need a vomit bag. Here are some examples from the New York Times,

He was constantly nauseated, so much so that he lost 50 pounds. In the place of public transport, fleets of private vans career from stop to stop with their hapless, nauseated passengers.

Nauseous, on the other hand, should be reserved to mean causing that feeling, not having it. But it's used so often now to mean "feeling sick," that dictionaries define it that way. Here's how to use the word if you want to tuck in your shirt and be proper:

It does not contain iodine, but is said to possess all the therapeutic qualities of cod-liver oil without its nauseous taste. (Encyclopedia Britannica)

But more examples look like this:

She was too nauseous to keep down her food. (BBC)

In his Modern American Usage, Garner labels this usage as "ubiquitous" but held on to by "die-hard snoots." The Oxford English Dictionary calls this usage of nauseous common. Another distinction is that nauseated can be used to describe "sick in the stomach" and nauseous for "sickening to think about."

Sticklers will keep the distinction. If you feel nauseated after thinking about this nauseous distinction, then you're on the right track.