The English language has always changed with use, so these days most dictionaries treat ‘nauseous’ and ‘nauseated’ as synonyms for one another. But this wasn’t always the case.
Traditionally, ‘nauseous’ and ‘nauseated’ had distinct meanings. You may, therefore, want to keep this in mind for academic work, especially if you’re writing about medicine.
Nauseous (Causing Nausea)
‘Nauseous’ is an adjective which originally meant ‘causing sickness or disgust’. Used this way, it applies to something that makes someone feel sick:
The nauseous smell of rotting meat made me want to vomit.
However, in modern English it’s also widely used to refer to feelings of sickness or disgust:
The smell of rotting meat made me nauseous.
Either of these is acceptable most of the time, but the first definition may be considered more technically correct in some settings.
Using the ‘technical’ definition, the second sentence above would imply that the smell of rotting meat turned the speaker into something that causes disgust or sickness!
Nauseated (Experiencing Nausea)
The technically-correct term for feeling sick or disgusted is ‘nauseated’. For instance, if we found violent horror films hard to watch, we might say something like:
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Watching The Texas Chainsaw Massacre Part 2 left me nauseated.
It’s easy to remember how this term should be used as long as you recall that ‘nauseated’ is the simple past tense of the verb ‘nauseate’, which means ‘to cause disgust or vomiting’.
Nauseous or Nauseated (or Nauseating)?
As mentioned above, most of the time ‘nauseous’ and ‘nauseated’ can both be used to mean ‘feeling sick or disgusted’. The important thing is to use terminology consistently.
Nevertheless, in some contexts, or if you want to impress pedants, it makes sense to maintain a distinction between these terms. In such cases, ‘nauseous’ should be used to mean ‘causing sickness or disgust’, while ‘nauseated’ would mean ‘feeling sick or disgusted’.
If you want to avoid confusion, moreover, you could use the adjective ‘nauseating’ to mean ‘causing sickness or disgust’, since this term only has one meaning. Remember:
Nauseous = Causing sickness or disgust (formal); or feeling sick or disgusted (informal)
Nauseated = Feeling sick or disgusted
Nauseating = Causing sickness or disgust