Essential oils should not be used ________ by estheticians.

Essential oils possess potent properties that can be beneficial for skin and aromatherapy applications. However, their use is highly concentrated and can lead to adverse effects when ingested. Estheticians are trained primarily in topical applications, focusing on skin health and external aesthetics. Internal use of essential oils is typically recommended only when under the guidance of a qualified healthcare professional, as it can pose risks such as toxicity or adverse reactions. Therefore, emphasizing the importance of safety and proper practices in esthetics, internal use is strictly advised against, making this the correct choice.