The International Fragrance Association (IFRA) recommends that Wormwood oil be limited to 0.4% (about 2.4 drops per ounce of any other carrier oils) for leave-on products like ointments, creams, and lotions. There is no restriction for body washes, shampoos, soaps, and other wash-off products because the oil does not remain on the skin.
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Wormwood oil is considered as a narcotic oil when used in higher doses. The adverse effects of Wormwood oil is attributed to the presence of chemical constituent thujone, which contributes to about 50 percent of the oil. Thujone containing oils are restricted prevalently, mainly due to the concern of its psychotropic, nephrotoxic and neurotoxic effects. It may cause skin irritation, redness, eruptions in sensitive skin and dermatitis.
Studies witness the excitation of the autonomic nervous system due to convulsions and binding of the GABA receptors caused due to α and β-thujone. It is also a known fact that high amount of thujone and its metabolites can induce convulsions, unconsciousness, seizures, gastrointestinal cramps, insomnia, tremors, dizziness, hallucinations, vomiting, headaches, retention of urine, renal lesions, congestive heart failure and rhabdomyolysis (collapse of muscular fibers).
Wormwood should never be used internally as there are numerous studies witnessing the severe toxic effects of ingesting this oil and few cases include absinthism and the acute renal failure due to rhabdomyolysis in a person who drank about 10 ml of Wormwood oil; In an another case, the intake of 60 ml of Wormwood oil ended up in seizure, altered mental faculty, attention disability, rhabdomyolysis and hyperthermia.
Blend Wormwood oil in mild carrier oils like Coconut oil or Avocado oil before topical use as pure and organic essential oils are highly concentrated substances and can harm the skin surface if used in a concentrated form. The safe level of dermal use for this oil is 0.4% by IFRA (International Fragrance Association).
The PanAfrican Medical Journal study talks about the burning effects of Wormwood herb, where a 50 year old woman reported a severe pain in the facial skin with a dry and delicate erythema in the cheeks and forehead classified as an initial degree burn.
With thujone being the toxic component, using Wormwood oil can act as a threat to pregnancy as it may affect the health of the uterus with its emmenagogue and abortifacient effects and is good to continue avoiding this oil during lactation as well.
It is advisable to restrict the use of this oil on children as they have a sensitive skin. Wormwood oil can also cause allergic reactions on the skin for those who are susceptible to plants of the Compositae/Asteraceae family like the Marigold, Ragweed, Chrysanthemum etc.,
Reference Links Substantiating the Possible Skin Issues of Wormwood Oil: