We generally recommend doing a patch test with any product you apply to your skin before using it. You find instructions for a patch test inside your Beginners Guide to Making Haircare at Home. There are essential oils which can cause alergic reactions or irritate the scalp but it's hard to predict which ones would cause this for you, that's why a patch test is important. As a general rule for sensitive skin and scalp types, stay away from: citrus oils, including lemon, lime, tangerine, grapefruit, mandarin, and bergamot oils. Other oils to avoid are camphor oil as well as mint oils such as peppermint, wintergreen, pennyroyal, and balm mint.