List of Harmful Essential Oils

Essential oils are commonly used in aromatherapy and other forms of alternative treatments for their healing properties. Some essential oils, however, can be harmful and even toxic, especially when consumed orally or applied directly to the skin. Caution should be used when handling harmful essential oils.As a general rule, never take undiluted essential oils by mouth, MedlinePlus cautions.

Tea tree oil is known for its woody, earthy, medicinal smell, and has been traditionally used for its antiseptic and antifungal properties. Tea tree oil might be effective in treating fungal infections such as athlete's foot and nail infections, MedlinePlus reports. MedlinePlus warns that tea tree oil can be dangerous if consumed, causing diarrhea, nausea, severe rash, confusion, stomach pain and coma.

Exercise caution as well when applying directly to the skin. Never use tea tree oil along with lavender oil topically on young boys; it can have hormonal side effects such as increasing breast size in young boys, a condition known as gynecomastia. Minor irritation such as allergic contact dermatitis can occur. Additionally, cases of blistering rashes have been reported after tea tree oil has made contact with the skin, especially in people with skin conditions such as eczema.

Peppermint has many healing properties, ranging from calming an upset stomach, to easing tension headaches and alleviating the symptoms of irritable bowel syndrome. The University of Maryland Medical Center, however, points out that peppermint oil should never be used on an infant or small child; if applied facially, peppermint oil can cause life-threatening breathing problems in infants. Peppermint oil should be used with care by those taking medications such as cyclosporine, drugs that treat diabetes, or drugs used to treat high blood pressure.

Rosemary essential oil is used in products which stimulate hair growth, and is a remedy for indigestion, muscle and joint pain, and alopecia, a type of hair disease. Rosemary oil can be toxic if ingested, and should never be used by people with high blood pressure, ulcers, Crohn's disease or ulcerative colitis. The University of Maryland Medical Center advises that rosemary can have interactions with antiplatelet and anticoagulant drugs, ACE inhibitors, diuretics and lithium, and can interfere with certain diabetes medications as well.

A popular ingredient in cold and flu medications, eucalyptus has been used to treat diabetes, relieve breathing problems and congestion, and as an arthritis remedy. Despite its multitude of uses, eucalyptus oil should never be ingested by children, and it should be avoided by pregnant or lactating women. The University of Maryland Medical Center points out that eucalyptus oil can trigger an asthma attack, so it should be used with caution by asthmatics. Eucalyptus oil should be avoided if you are taking 5-fluorouracil.