Yes, essential oils are all natural and are an important and effective part of alternative medicine.  But not everything that is natural is good for you!

In a previous guide, "How to Choose Essential Oils that Actually Work," I have explained what to look for when buying essential oils - to summarize, purity, authenticity, and being clearly labeled with the latin name of the plant. 

In this guide I will review important safety information in general, plus which essential oils to avoid using on your skin, during pregnancy, and when you have high blood pressure, epilepsy, kidney disease, or diabetes.  This is meant to be a resource for you to print and refer to as needed (it is pretty dry reading!).

This list is fairly exhaustive, not because these essential oils are common at retail stores, but because they are common with suppliers of pure, authentic, latin-named oils.

Remember, most essential oils are safe when used as directed.  Don't let this information scare you off from all the wonderful benefits of using essential oils.  There are still well over 100 essential oils that you can use without concern!

As a reminder, never take essential oils internally unless you are specifically directed to by a certified and registered Aromatherapist.

Never, Never, NEVER Use These Essential Oils

Unless under the supervision of a certified and registered Aromatherapist, do not even be tempted to smell these oils!

• Ajowan
• Trachyspermum copticum - dermal (skin) irritant, possibly irritating to mucous membranes (ex. the membranes inside your nose), and must not be used during pregnancy
• Almond, Bitter
• Prunus dulcis - contains prussic acid, also known as hydrocyanic acid or cyanide, is a well-known poison; also contains the moderately toxic benzaldehyde
• Allspice, Leaf and Berry
• Pimenta dioica - dermal (skin) irritant; and, due to the eugenol content, very irritating to mucous membranes (ex. the membranes inside your nose)
• Anise, Star
• Illicium verum - although it does not seem to be toxic when used as recommended, if you use too much it can slow down your circulation and lead to cerebral (brain) disorders, so it is best to avoid it alltogether
• Many retail stores sell star anise essential oil despite the safety concerns - do not assume that just because you see it on the retail shelf that it is safe!
• Aniseed
• Pimpinella anisum - causes dermatitis, and, like star anise, if you use too much it can slow down your circulation and lead to cerebral (brain) disorders, so it is best to avoid it alltogether
• Arnica
• Arnica montana - the essential oil is highly toxic, however tinctures or homeopathic remedies are safe and very useful
• Basil, Exotic
• Ocimum basilicum - these safety cautions only apply to exotic basil, and not to French basil (although they have the same latin name, they are made up of dramatically different constituents); moderately toxic, dermal (skin) irritant, and may be carcinogenic (causes cancer)
• Birch, Sweet
• Betula lenta - although the methyl salicylate contained in sweet birch isn't exactly toxic, if you use too much it can be very harmful and so best to avoid altogether
• Boldo
• Peumus boldus - extremely toxic
• Broom, Spanish
• Spartium junceum - toxic due to the sparteine content
• Buchu
• Agothosma betulina - should not be used during pregnancy or be used around anyone who is pregnant
• Calamus
• Acorus calamus var. angustatus - an oral toxin, plus strongly suspected of being carcinogenic (causes cancer)
• Camphor, Brown and Yellow
• Cinnamonomum camphora - brown and yellow camphor contain safrol and are toxic and carcinogenic (causes cancer); white camphor is relatively non-toxic
• Cassia
• Cinnanomum cassia - dermal (skin) toxin, irritant, and sensitizer; and very irritating to mucous membranes (ex. the membranes inside your nose)
• Watch out for cassia essential oil - I don't know why, but it is popularly used by handcrafted soapmakers (who are unaware of the risks), especially in abrasive (exfolliating) soaps
• Cedarwood, Texas or Virginian
• Juniperus ashei and Juniperus virginiana, repectively - use Atlas cedarwood instead, as these two kinds of cedarwood can cause acute local irritation
• Chervil
• Anthriscus cerefolium - strong irritant and toxin, highly suspected of being carcinogenic (causes cancer)
• Cinnamon, Bark
• Cinnamomum zeylanicum - the leaf oil is relatively non-toxic, but the bark essential oil is a dermal (skin) toxin, irritant, and sensitizer; it is also very irritating to mucous membranes (ex. the membranes inside your nose)
• I have heard of beauty magazines publishing homemade lipbalm recipes calling for the spice cinnamon - cinnamon spice is made from the bark, and I have had to explain to people that this is why the lip balm made their lips burn!
• Clove, Leaf and Stem
• Syzygium aromaticum - dermal (skin) irritant and very irritating to mucous membranes (ex. the membranes inside your nose); clove bud essential oil is a less toxic alternative because it contains less eugenol (but still use cautiously)
• Costus
• Saussurea costus - severe dermal (skin) sensitizer (causes allergic reactions in people who do not have allergies)
• Deertongue
• Carphephorus odoratissimus - anecdotally has caused liver damage and hemorrhages, plus the lactones can cause dermal irritation
• Elecampane
• Inula helenium - although it is non-toxic, it is a severe dermal (skin) sensitizer (in clinical tests it caused "extremely severe allergic reactions" in 23 out of 25 volunteers)
• Fennel, Bitter
• Foeniculum vulgare - causes sensitization (allergy-like symptoms) and should never be used on the skin - if you must use fennel, make sure it is sweet fennel (not bitter fennel)
• Horseradish
• Armoracia rusticana - an oral toxin, dermal (skin) irritant, and very irritating to mucous membranes (ex. the membranes inside your nose)
• Jaborandi
• Pilocarpus jaborandi - an oral toxin, skin irritant, and will cause medically dangerous spontaneous abortions in pregnant women
• Melilotus
• Melilotus officinalis - in 1953 many countries, including the USA, banned Melilotus from use due to toxicity levels
• Mugwort
• Artemisia vulgaris - an oral toxin due to high thujone content, and will cause medically dangerous spontaneous abortions in pregnant women
• Mustard
• Brassica nigra - mustard essential oil is considered to be one of the most toxic of all essential oils; an oral toxin, dermal (skin) toxin, and very irritating to mucous membranes (ex. the membranes inside your nose)
• Narcissus
• Narcissus poeticus - besides causing headaches and vomiting, Narcissus profoundly effects the nervous system and can cause paralysis
• Oregano, Common and Spanish
• Origanum vulgare and Thymus capitatus, respectively - dermal (skin) toxin and irritant, and very irritating to mucous membranes (ex. the membranes inside your nose)
• Pennyroyal
• Mentha pulegium - an oral toxin that also causes medically dangerous spontaneous abortions in pregnant women
• Pine, Dwarf
• Pinus mugo var. pumilio - dermal (skin) irritant that causes sensitization (allergic reactions in people who do not have allergies)
• Rue
• Ruta graveolens - the essential oil is highly toxic, however homeopathic remedies are safe and very useful; specifically  an oral toxin, dermal (skin) irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and causes medically dangerous spontaneous abortions in pregnant women
• Sage, Common
• Salvia officinalis - an oral toxin that causes medically dangerous spontaneous abortions in pregnant women; although they should be avoided in pregnancy, Spanish Sage and Clary Sage are otherwise non-toxic alternatives
• Santolina
• Santolina chamaecyparissus - an oral toxin
• Sassafras
• Sassafras albidum - highly toxic, highly irritant, carcinogenic (causes cancer), and causes medically dangerous spontaneous abortions in pregnant women
• Savine
• Juniperus sabina - an oral toxin and dermal (skin irritant), it is banned from sale in many countries because of its effect on circulation and for causing medically dangerous spontaneous abortions in pregnant women
• Savory, Summer and Winter
• Satureja hortensis and Satureja montana, respectively - dermal (skin) toxin and irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and causes medically dangerous spontaneous abortions in pregnant women
• Tagetes
• Tagetes minuta - strongly suspected of causing dermatitis and considered generally harmful
• Tansy
• Tanacetum vulgare - an oral toxin due to high thujone content, and causes medically dangerous spontaneous abortions in pregnant women
• Tarragon
• Artemisia dracunculus - the essential oil is moderately toxic due to methyl chavicol, but more importantly it is suspected to be carcinogenic (causes cancer) and must not be used during pregnancy
• Thuja
• Thuja occidentalis - the essential oil is highly toxic, however homeopathic remedies are safe and very useful; an oral toxin, to the extent of being poisonous, and causes medically dangerous spontaneous abortions in pregnant women
• Thyme, Red, Serpolet (wild), and "thymol" and "carvacrol" types
• Thymus vulgaris - the lemon and "linalol" types are generally less toxic and considered safe for use on the skin and with children (white thyme is often adulterated and should be avoided); contain large amounts of toxic phenols, dermal (skin) irritant, very irritating to mucous membranes (ex. the membranes inside your nose), and must be avoided during pregnancy or if you have high blood pressure
• Tonka
• Dipteryx odorata - an oral and dermal (skin) toxin because of its coumarin content
• Turpentine
• Pinus palustris and other Pinus species labeled as "Turpentine" - turpentine essential oil is an environmental hazard and a marine pollutant
• Wintergreen
• Gaultheria procumbens - the essential oil is a toxin, irritant, and sensitizing; in fact, wintergreen essential oil is considered environmentally hazardous and a marine pollutant
• Many retail stores sell wintergreen essential oil despite the safety issues - do not assume that just because you see it on the retail shelf that it is safe!
• Wormseed
• Chenopodium ambrosioides var. anthelminticum - a very toxic essential oil that can explode when heated due to ascaridole content
• Wormwood
• Artemisia absinthium - extremely toxic, and will cause medically dangerous spontaneous abortions in pregnant women

Additional Essential Oils to Avoid Using on the Skin

These can be used on the skin at half strength (make sure to confirm that when using already-made aromatherapy products).  Never apply these neat (directly and undiluted) to the skin).

• Basil, Sweet (or Lemon Balm or Melissa) - Melissa officinalis
• Black Pepper - Piper nigrum
• Borneol - Dryobalanops aromatica
• Cajeput - Melaleuca cajeputi
• Caraway - Carum carvi
• Cinnamon, Leaf - Cinnamomum zeylanicum
• Clove, Bud - Syzygium aromaticum
• Cornmint - Mentha arvensis
• Eucalyptus, Common - Eucalyptus globulus
• Garlic - Allium sativum
• Ginger - Zingiber officinale
• Lemon - Citrus limon
• Parsley - Petroselinum sativum
• Peppermint - Mentha piperita
• Pine Needle (Scotch and longleaf) - Pinus sylvestris and Pinus palustris, respectively
• Thyme, White - Thymus vulgaris, specifically "White"
• Tumeric - Curcuma longa

Additional Essential Oils to Avoid During Pregnancy

• Angelica - Angelica archangelica
• Basil, Sweet (or Lemon Balm or Melissa) - Melissa officinalis
• Bay Laurel - Laurus Nobilis
• Calamintha - Calamintha officinalis
• Cedarwood, Atlas - Cedrus atlantica
• Celery Seed - Apium graveolens
• Cinnamon Leaf - Cinnamomum zeylanicum
• Citronella - Cymbopogon nardus
• Clary Sage - Salvia sclarea
• Clove Bud - Syzygium aromaticum
• Cumin - Cuminum cyminum
• Fennel, Sweet - Foeniculum vulgare
• Hyssop - Hyssopus officinalis
• Juniper Berry - Juniperus communis
• Labdanum - Cistus ladanifer
• Lovage - Levisticum officinale
• Marjoram, Sweet - Origanum marjorana
• Myrrh - Commiphora myrrha
• Nutmeg - Myristica fragrans
• Parsley - Petroselinum sativum
• Snakeroot - Asarum canadense
• Sage, Spanish - Salvia lavandulifolia
• Thyme, White - Thymus vulgaris, specifically "White"

Also avoid these essential oils during the first four months of pregnancy

• Peppermint - Mentha piperita
• Rose - all Rosa species, including Rose x centifolia and Rosa damascena
• Rosemary - Rosemarinus officinalis

Additional Essential Oils to Avoid if You Have High Blood Pressure

• Hyssop - Hyssopus officinalis
• Rosemary - Rosemarinus officinalis
• Sage, Spanish - Salvia lavandulifolia
• Thyme, All Varieties - Thymus vulgaris

Additional Essential Oils to Avoid if You Have Epilepsy

• Clary Sage - Salvia sclarea
• Fennel, Sweet - Foeniculum vulgare
• Hyssop - Hyssopus officinalis
• Rosemary - Rosemarinus officinalis
• Sage, Spanish - Salvia lavandulifolia

Additional Essential Oils to Avoid if You Have Kidney Disease

• Juniper Berry - Juniperus communis

Additional Essential Oils to Avoid if You Have Diabetes

• Angelica - Angelica archangelica

Want to Learn More?

To learn more about finding real essential oils, go to "How to Choose Essential Oils that Actually Work."

To learn about candles and aromatherapy, go to "Are Soy and Natural Candles Really Natural?"

I STRONGLY recommend the book, "The Illustrated Encyclopedia of Essential Oils," by Julia Lawless if you want to use essential oils. 

Please check out my other reviews, too:

"How to Make Your Own Chandelier Earrings Easily"

"Make Your Own Chocolate Candies"

"Soap Making Secret - How to Make Your Glycerin Soaps Last Longer"

"The Secret of Using Rutin to Trim Your Waist"