Among the many considerations to be noted before going full steam into the practice of aromatherapy is that of the oils themselves. Certain oils are irritants and may have an ill effect in external application, or even in use as inhalants. However, it is a caution well heeded to acknowledge that, when dealing with different people, they may carry different sensitivities with them. This includes, but is not limited to, allergies. For example, there was one family I worked with who had an apparent inherited sensitivity to canola oil.
Usually considered a harmless oil, in the case of this family the negative effects created in using this oil could far outweigh any benefits that might have been realized. For the responsible worker of fragrance, then, the bottom line is to know who you are working with as well as the properties of the oil blends you employ.