Erythromycin ointment cannot be used to treat mouth ulcers. The composition of erythromycin ointment includes erythromycin, paraffin oil and yellow petroleum jelly, etc. It belongs to the topical antimicrobial drugs, which can kill the sensitive bacteria, and is mainly used for the treatment of pustules, ulcers and co-infections caused by the infection of sensitive bacteria. The drug is for external use only and cannot be used on the eyes and other mucous membranes (e.g., mouth, nose, etc.), so it cannot be used to treat mouth ulcers. The most common adverse reactions to the drug include burning dryness, erythema, itching of the local skin mucous membranes. It should be noted that the drug is contraindicated in people who are allergic to it, and allergists use it with caution. Erythromycin ointment application needs to strictly follow the doctor’s instructions, so as not to cause adverse consequences due to the wrong medicine.