Essential oils are highly concentrated and can be harmful if not used carefully, so it is important to do a lot of research and learning before using them. I also feel that it is important to err on the side of caution and to not use essential oils haphazardly. I am not an expert, so you need to do your own research and consult your physician if you have a medical condition. Essential oils are not going to replace proper medical care.
Some of the safety guidelines are:
- Never use undiluted on the skin
- be careful and watch for allergic reactions
- Some essential oils are phototoxic and can cause problems when applied and then you are exposed to sunlight
- Some essential oils should be avoided during pregnancy or when you have another health condition such as asthma or epilepsy
- Better to use less. They are very concentrated and more is not better, it could be dangerous.
- Always keep them away from children. They are very concentrated and can be dangerous when not used properly.
- Essential oils should not be taken internally without proper guidance and knowledge of safety.
- Keep away from fire as they are flammable.
There is a pretty comprehensive book on safety Essential Oil Safety: A Guide for Health Care Professionals-, 2e.