Hi Payal Welcome to the HCM There s some evidence that food allergies, viral infections, and dietary deficiencies (in particular, not getting enough iron, folic acid, zinc, or B12) can trigger oral ulcers. Ulcers almost always go away on their own, usually in a week to ten days (although large ones can take longer). The pain often subsides in three or four days. So you may take the following measures to treat her and prevent future episodes: -Apply ice to the sore to help numb the area. If your baby s eating solid food, cold or frozen foods — such as ice pops — may also do the trick. But don t give her hot, spicy food or citrus, which could make her mouth hurt more. - Apply teething gel or cream, but apply it carefully as touching the sore can hurt. For a home remedy, combine 1 part hydrogen peroxide and 1 part water. Dab on the canker sore with a cotton swab. Follow by dabbing on a little milk of magnesia. Do this three or four times a day to soothe the area and help it heal. -Give her pain relievers like acetaminophen or ibuprofen - continue multivitamin and give her a balanced diet with regular fluids. Hopefully this will help you. Take care