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Commonest cause of chronic nausea is acidity. Take omeprazole (empty stomach in morning) and antacid gel and see if it helps. Liver and kidney dysfunction is another cause. Hence kidney and liver function tests should be done. Other causes are diabetes and hypothyroidism. Electrolyte imbalance too can cause nausea. IBS, gluten or lactose intolerance, parasites in gut too can be the cause. A stool test should be done for worms, giardiasis etc. Chronic hepatitis or chronic infection of the gall bladder are the other possibilities. Also if nothing is found then food challenge test to detect food intolerance should be done. Keep a food diary and see if there is a co-relation between what you eat and symptoms that appear.
Please consult your PCP for primary examination followed by proper referral.