Sometimes repeated exposure to something can increase your risk of developing an allergy to ie, for example a very small percentage of bees keepers who developed a propolis allergy after 9 years of handling it every day. 9 years of regular, daily exposure to a product is a long time to be constantly exposed to something. Most people would only use a product when they needed to. Unlike antibiotics, the use of essential oils such as Manuka Oil has been shown to not cause resistance or tolerance. That means it's effectiveness does not decrease over time.