Oranges, grapefruits, and lemons are tasty as both fruits and juices, and are packed with vitamin C. But their acid content can erode enamel, making teeth more vulnerable to decay. Even squeezing a lemon or lime into water adds acid to a drink. Plus, acid from citrus can be bothersome to mouth sores. If you want to get a dose of their antioxidants and vitamins, eat and drink them in moderation at mealtime and rinse with water afterward.