You had reactions to tetracycline and ampicillin, which are antibiotics, but chlorhexidine mouthwash is not an antibiotic; it is an antiseptic used locally for gum problems. It is generally safe for people with antibiotic allergies, though it may cause temporary taste changes, mild burning, or staining of teeth; true allergy is very rare. You can usually use it safely for gum pain, but if you develop swelling, rash, or breathing difficulty, stop and seek medical help.