Can Penicillin help to cure mouth ulcers?

The short answer is 'no'. It depends on what has caused the mouth ulcer. Common causes of mouth ulcers include local trauma (e.g. biting your lip), poorly fitted dentures and viral infection. Penicillin will not help in any of these situations. Penicillins only work to treat bacterial infections. A bacterial infection is unlikely to be the cause of mouth ulcers.