It is possible in some cases to have allergic reactions to certain essential oils, which is why it is important to test them beforehand on a small part of the skin. In addition, some essential oils, particularly citrus zest, are photosensitizing. The latter contain in their compositions furocoumarins which promote the production of melanin by the skin and can cause sunburn and brown spots. It is recommended to avoid any contact with the sun within 12 hours of using one of these essential oils.
Among other side effects, some essential oils have a low to moderate neurotoxicity due to the presence of ketones. These can be harmful to the nervous system and especially to epileptics and young children, so they must be used occasionally and strongly diluted before any use, or they can be used externally, since the skin filters the ketones and uses them for their regenerative quality of the epidermis.
Finally, some essential oils are dermocaustic and are not recommended for use on the skin, which can cause slight burns and irritation. These products should be used in moderation and particularly diluted (less than 1%) in vegetable oil to avoid any inconvenience.
In short:
Essential oils are powerful, they require in every cases to take adequate precautions. They can sometimes have side effects, due to the user’s lack of knowledge and misuse. It is important to test them on a small area of skin before using them on a larger scale. And as with all products, if you follow the right dosage recommended by aromatherapy professionals, there is no danger.