Prilosec (omeprazole) is a proton pump inhibitor (PPI) used for the treatment of conditions such as ulcers, gastroesophageal reflux disease , and Zollinger-Ellison syndrome, which are all caused by stomach acid. Prilosec is available in generic form. Side effects of Prilosec include: headache, nausea, vomiting, diarrhea, stomach pain, gas, constipation, fever, or ; cold symptoms (stuffy nose, sneezing, and sore throat). The recommended adult oral dose of Prilosec ranges from 20 mg to 60 mg once daily, depending on the condition being treated.