Flawed statistics in a scientific review in the journal of the Swedish Medical Association in 2011 greatly exaggerated the frequency of contact allergy from herbal dermal products, particularly tea tree oil, as pointed out in a letter to the editor by four Australian microbiologists. I C contacted the review authors who regretfully acknowledge the error.
Three Swedish physicians wrote the original article from 2011. It was based on a scientific literature review for four herbals: tea tree oil, Herstat, Oleum salvum and Efabene.
Admit mistake
Tea tree oil was said to involve the largest risk of adverse reactions. Contact allergy prevalence for patients using the herbal was noted as varying between 0 and 26 percent in the various studies. However, now one of the authors admits in an e-mail to I C that the group made a mistake.
Apparently, the error occurred when 13 cases (26 percent) were incorrectly recorded in the wrong column in reporting one of the studies. The correct frequency of contact allergy in the study in question should actually have been 0 (zero).
Possible conflict of interest
While the Australian microbiologists from University of Western Australia, Crawley seem to have been correct, their potential conflict of interest should be noted. They have previously received research funding or consultancy fees from the Australian government and from manufacturers of essential oils.
Obviously, herbals may cause serious side effects, including contact allergy, and it is important that these be reported, the letter says. The problem is that the side effects of tea tree oil have been grossly overstated in the review article. The Australian commentators raise several objections.
Exaggerated side effects
The Australian researchers assert that studies demonstrating lower prevalences of contact allergies for tea tree oil (0,3–2,7 percent) have been excluded from the review. The study referred to as representing the higher limit of dermal side effects has been completely misinterpreted, they say. The prevalence in that study should instead have been reported as zero.
On examining the original article, I C concurs in the interpretation of the Australian team, that the reported prevalence of contact allergy to tea tree oil is zero. As previously mentioned, the authors of the review article have confirmed by e-mail to I C that they made a mistake.
More sources of error
There is also an error in the study reporting the second highest prevalence of contact allergy to tea tree oil (11 percent). Study participants were recruited among volunteers instead of consecutive patients visiting a specialist physician. It is well known that the prevalence of positive reactions to a prick test normally is higher in selected patients.
Storage entails oxidization
Furthermore, elevated prevalence rates for contact allergy of tea tree oil may be due to the fact that certain of the oils used in the studies have been stored for up to eight years and so have oxidized. In the process, peroxides are formed, considered to be the primary cause of skin-related side effects in using tea tree oil.
Hence, reports about the prevalence of contact allergy to tea tree oil should also contain data on the whether or not the oils have oxidized or not, the Australian scientists suggest.
Wrong about lethal side effects
Finally, the microbiologists maintain that the information given concerning lethal side effects of herbal (11 deaths 1987-2000) were misleading. Other CAM preparations were included in the statistics.
Only in five cases may there according to WHO criteria have been a connection between side effects and CAM preparations, and not a single case resulting in death has been related to tea tree oil, Herstat, Oleum salvum or Efabene (the four herbals studied in the review from 2011).
Lower side effect frequency
The correct lower side effects rates for tea tree oil in various studies are 0–4,3 percent and for other herbals for external use 0–6,4 percent according to the critics. This lower frequency for contact allergies resulting from the use of tea tree oil are in accordance with the conclusion in the evidence-based US database Natural Standard (www.naturalstandard.com).
Subscribers of that service will be able to learn that the use of tea tree oil is safe for first-time users without known allergy to tea tree oil or other plants of the family Myrtaceae. However, users are asked to be careful, particularly after repeated use of the oil, since contact allergies from tea tree oil may occur.
David Finer & Johanna Hök
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Brenan JA Evaluation of patch testing in patients with chronic vulvar symptoms. Australas J Dermatol. 1996 Feb;37(1):40-3.
Natural Standard Database (hämtad 2012-08-13).
