Overview

As with any living organism, plants depend on various chemical reactions to function properly. The interactions between substances result in secondary compounds, which are stored in leaves, flowers, stems, and roots of the plant. Essential oils are plant extracts that carry volatile herbal compounds. Most parts of the plant can be distilled or mechanically pressed to obtain their oils.¹

Although traditionally used in many cultures for a long time, the interest in associating holistic approaches to healthcare, including essential oils, has risen in the past decade. They can be found in food, fragrances, detergents, and lotions.² The most commonly used essential oils in aromatherapy, for example, are obtained from tea tree, peppermint, patchouli, ylang-ylang, lemon, lemongrass, eucalyptus and orange.³

Essential oils have high and diverse concentrations of chemicals, which can give them pharmacological properties. Their safety and efficacy are not well established and scientific proof of the benefits of plant extracts on health issues is still scarce, especially compared to medications. some users may put themselves at risk, by naively substituting prescribed medications for oil treatments.² Because of its complex and varied combination of chemicals, essential oils may present side effects, so being aware and educated about them is important to implement their use.

Common side effects of essential oils

Skin irritation and allergic reactions

Essential oils are composed of a mix of even hundreds of substances, a reaction can be related to any of these chemicals. When in direct contact with the skin, essential oils can cause irritations and hypersensibility, manifesting in redness, itching, scaling and rashes. The user can even develop dermatitis from the allergic reactions after frequent or sustained contact with the skin. Tea tree, lavender and peppermint oils, to name a few, are not only among the most used but also known to be commonly allergenic. Symptoms usually appear after 24 hours from exposure and can spread from the skin surface of contact.³

People who keep frequent contact with these substances (massage therapists, hairdressers) or already have pre-existing skin allergies are the most susceptible to developing irritation from essential oils.³ 

Patch tests are simple, controlled and efficient in diagnosing skin irritations and allergies. Small patches with different substances are applied to the skin and observed for reactions. Results guide the patient on what to avoid contact with.³

Respiratory issues

For the wide variety of volatile compounds, essential oils are often used in aromatherapy or just for their fragrance. These compounds are then carried through the respiratory system being in contact with mucosa, and lungs and holding easy access to the bloodstream. Respiratory reactions can cause cough, nose and throat irritation and shortness of breath. Potentially allergenic substances close to sensitive areas of the body can be a risk, especially for individuals with asthma and pre-established allergies. Widely diffused beliefs about the benefits of inhaling essential oils for treating respiratory issues can be harmful and should be followed and advised by a specialized doctor.⁴

Headaches and dizziness

Although often used as an alternative to treating headaches and dizziness, essential oils may trigger headaches, migraines and dizziness in some individuals. These individual reactions can be due to strong scents, highly concentrated oil, poor quality, and a mix of compounds. Peppermint, eucalyptus and lavender oils are especially reported to cause such discomfort. 

Serious side effects

Hormonal imbalance

Lavender and tea tree oils, two of the most commonly used in a variety of forms and products, have been linked to hormonal imbalances in adolescents. Both males and females in their early teenage years have been studied for hormonal effects from the use of these oils from daily products. Evidence points especially to abnormal breast development on both, raising the suspicion that compounds in these essential oils can increase estrogenic and anti-androgenic activity. It was also observed that cutting the use of products with lavender and tea tree oil regressed the breast tissue to the previous state. These findings are only exploratory and require further investigation, but reveal the possibility of essential oils interfering in the hormonal balance.⁶

Toxicity

Ingestion of essential oils can cause toxicity. They are composed of a variety of chemicals, often alcohol, potentially being poisonous.⁷

Examples of toxic oils by ingestion

• Pennyroyal: toxic to the liver⁷
• Wintergreen: extremely harmful depending on the dose for being highly concentrated. A small amount can cause the effects of a large portion of adult aspirins⁷
• Nutmeg:  it can cause hallucinations and coma⁷
• Eucalyptus: can cause seizures⁷
• Camphor: can cause seizures, especially in children⁷

Signs of toxicity 

• Nausea
• Vomiting
• Seizures

Phototoxicity

Exposure to sunlight may cause skin sensitivity, called phototoxicity. Sun rays can interact with photoreactive chemicals, changing their structures. In the case of essential oils in contact with the skin, these alterations may transform them into cytotoxic substances (harmful to cells). Sunbathing with certain essential oil residuals on the skin can result in mild to severe burns, causing irritation, redness, itching and oedema. A variety of citrus oils are known to be photoreactive,  Their photo reactiveness can be dependent on the way they were extracted.^8,9

Essential oils which commonly cause phototoxicity 

• Bergamot
• Lemon
• Lime
• Lemon verbena
• Fig leaf
• Grapefruit
• Cumin
• Mandarin leaf

Precautions for using phototoxic oils

Some essential oils, like lime, can be photoreactive if extracted by distillation, but safe if extracted by mechanical pressure. Others can be diluted to a safe level if the carrier oil used for that is not photoreactive. These choices can be complicated for people who are not well-educated in the use of essential oils. Prioritizing safety, it is better to look for non-photoreactive oils or cover the skin when being exposed to the sun.⁹

Safety guidelines for using essential oils

• Proper Dilution: follow dilution ratios for topical application with  carrier oils to reduce the risk of irritation
• Avoiding Ingestion
• Safe Storage and Handling: prevent accidental exposure or ingestion
• Awareness: being informed about essential oils and their effects

Summary

Essential oils are not only traditionally used in many cultures, but have been gaining attention among new enthusiasts. Plants are rich in a wide variety of chemicals, These compounds give essential oils the potential to provide health and well-being support. Some may use them for the pure pleasure of the fragrances. Others are supporters of their medicinal use, mainly known by empiric accumulated knowledge and tradition. A scarce range of studies focus on exploring the properties of essential oils, so they are still not established and regulated as medication. Their use must come with caution and awareness. Misusing essential oils can cause skin irritation, allergies, burns, seizures, respiratory issues, headaches and many other symptoms. People with previous health issues, especially allergies and respiratory disorders, must follow a doctor's guide before using these plant extracts.