Cold sores result from infection with one of two types of the herpes simplex virus, usually type one. There is no cure for herpes, so people who contract the virus carry it for life, according to National Institutes of Health dermatologists Stephen E. Straus and Adriana R. Marques in the 2008 edition of "Fitzpatrick's Dermatology in General Medicine." However, not all infected people develop cold sores. Some people never develop them, while others experience recurrent bouts for reasons that doctors and scientists do not fully understand. Lemon juice may trigger new cold sore outbreaks and make existing outbreaks worse.
Scientists describe the chemical properties of foods in terms of pH. PH is a measure of the acidity or alkalinity of a substance. Highly acidic or alkaline foods corrode the tissues of the mouth, precipitating cold sore outbreaks for some people and making existing outbreaks worse. Food and Drug Administration data indicate that lemon juice, along with whole lemons, whole limes and lime juice, features a pH between 2 and 2.6, qualifying it as the most corrosive food substance.
Scientists describe the physical properties of foods in terms of state, such as solid, liquid or gas. The liquid state of lemon juice actually magnifies the corrosive effects of the acids in lemon juice. While solid lemons stick together, limiting contact between mouth surfaces and corrosive acids, lemon juice literally bathes mouth surfaces in acids. Increased exposure translates into increased damage that can cause new cold sores or make existing ones worse.
An 8-oz. glass of lemon juice contains over 150 percent of the daily value for vitamin C plus a variable quantity of antioxidant flavonoids. Both vitamin C and flavonoids appear to reduce the risk of cold sores by supporting the health of the immune system and inhibiting the growth of the virus that causes them. However, few people consume lemon juice in quantities sufficient to provide nutritional benefits because its acidity confers an unpleasantly sour taste.
MedlinePlus, a patient information service jointly maintained by the National Library of Medicine and the National Institutes of Health, recommends that people with cold sores steer clear of all citrus--not just lemon juice--as well as salty, spicy and hot foods. However, dietary strategies do not replace conventional medical treatment for cold sores or any other condition. People who experience severe, frequent or long-lasting—longer than two weeks—outbreaks should see a doctor or dentist. A doctor or dentist can rule out more serious conditions, such as oral forms of cancer or autoimmune disease, and determine whether prescription antiviral drugs are needed.
People with cold sores who wish to continue consuming lemon juice or lemons should consider alternative strategies. One such strategy is to follow lemon juice with neutral or alkaline foods such as grains, legumes, dairy products, meat, fish or poultry. Rinsing with over-the-counter, neutralizing dental rinses or a homemade solution of baking soda and water is another. Applying lip balms or mouth ointments to the areas of the mouth affected by cold sores puts a protective barrier between tissue and corrosive acids, but this only works if it done before the juice is consumed.