What should i do about nausea/sore throat caused by allergies?

Post nasal drip: Usually the sore throat with allergies is due to post-nasal drip irritating the throat. Getting better control of your allergies with nasal medication or, more effective still, allergen immunotherapy, would address the root cause. But in the short term, try a sinus rinse like a netipot and see if that can decrease the amount of mucus that drains into your throat.