To protect your stomach, your doctor may suggest switching to a different antibiotic class, like certain cephalosporins, or adding a stomach-protective medication like omeprazole or famotidine. For your chest congestion and mucus, a mucolytic like guaifenesin or acetylcysteine can help thin the mucus, making it easier to cough up without irritating your stomach. Always take your medications with food if directed, and stay well-hydrated. Please consult your prescribing physician before stopping or changing your antibiotic regimen so they can safely adjust your treatment plan to accommodate your antral gastritis.