There are certain essential oils that are better left alone. I got this information out of the aroma therapy book by Scott Cunningham and thought it was definitely worth putting up on my site! Certain essential oils are dangerous for pregnant women to even touch, and others can cause epileptic fits, I believe the more people aware of these possibilities the better!
 

Basil-This oil should not be used at all!

Bergamot-If used on skin and exposed to sunlight it can cause a severe sunburn!

Camphor-Prolonged inhalation can cause headaches.

Cinnamon bark and leaf-Skin irritant. Do not anoint or use in a bath.

Clary Sage-Not to be used with alcohol. Prolonged inhalation can cause headaches.

Clove-bud-leaf-stem-Skin irritant do not anoint or use in a bath.

Fennel-Bitter-Irritant, causes epileptic attacks, not to be used by pregnant women!

Frankincense-May irritate the skin

Hyssop-Causes epileptic attacks, possibly other problems, do not use if pregnant.

Lemon-Irritant, do not anoint or use in a bath.

Lemon Balm-Irritant, do not anoint or use in a bath.

Lemon Verbana-Irritant, do not anoint or use in a bath.

Marjorham-Not to be used by pregnant women.

Mugwort-Hazardous! Should not be used!

Myrrh-Irritant, do not use if pregnant.

Oregano-Irritant, do not use if pregnant.

Pennyroyal-Toxic! Do not use!

Rue-Dangerous do not use!

Sage-Toxic! Do not use!

Thyme-Toxic! Do not use!

Ylang Ylang-May cause headaches from prolonged exposure.
 

Now remember these are the Essential Oils not the herbs themselves!