I feel nauseous. / I have nausea.
NAU-ze-a — three syllables. 'Au' is like 'ow' in 'now'. The 's' is like a 'z'.
Common symptom — report it at the doctor's or to a pharmacist. Also useful on a boat, in a car, or after eating something bad.
'Ho la nausea' uses 'avere' with the definite article 'la'. Like 'ho la febbre', the article is required. 'Nausea' is an international medical term used identically in Italian. The related verb is 'avere la nausea' or 'sentirsi nauseato/a' (to feel nauseated).
Mi sento nauseato.
I feel nauseated. (male speaker)
Alternative using the past participle as adjective
Ho voglia di vomitare.
I feel like vomiting.
More direct — 'voglia di' + infinitive means 'feel like doing'
Ho lo stomaco sottosopra.
My stomach is upset.
Idiomatic — 'sottosopra' means upside down/all over the place
Food poisoning ('intossicazione alimentare') in Italy is treated primarily with rest, oral rehydration, and 'carbone vegetale' (activated charcoal) — widely available in Italian pharmacies without prescription. 'Fermenti lattici' (probiotics/lactic bacteria) are also commonly recommended. Severe cases requiring IV fluids go to 'pronto soccorso'.