Do Rennie (indigestion) tablets help soothe mouth ulcers (canker sores)?

First it is important you need to distinguish whether it is a cold sore ( simplex virus infection) or canker sore (aphthous ulcer). They both occur in the mouth, but have different presentation, etiology and different treatment choices. However, let's just ume what you have is really canker sores. My answer to your question is "yes." However, it does nothing to heal the ulcer. It temporarily reliefs the symptoms and that is about it. The etiology is rather vague. According to the literature, it can be any thing from nutritional deficiency to viral infection (strange). The best course of treatment is steroid application for about 5days.