Can antibiotics cause gastritis and dosage of nexium (esomeprazole) to treat it?

No: Antibiotics don't cause gastritis although some are harder on stomach than others. Erythromycin & clindamycin are known to be especially harsh on gut, former w/nausea & vomiting while latter w/diarrhea (although all are known to cause C diff diarrhea).There's no specific dose of Nexium (esomeprazole) or any other proton pump inhibitor to treat side effects.