Is your pet’s tick & flea product harmful to humans?

7th December 2022
Beryl Shuttleworth
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We’ve have spoken often about how harmful various tick and flea products are for your dog or cat. But are they harmful to humans?

There is a massive and very respected group in the US called the Natural Resources Defence Council (NRDC). Boasting 1.3 million members and various experts such as scientists, vets, doctors and lawyers, they are not lacking in credibility. This group looks thoroughly into various issues and one of the issues they have probed is tick and flea treatments for pets. (1)

I have looked into the chemicals most often used in South African products and summarised the findings here:

Piperonyl butoxide (PBO)

This chemical is not an insecticide as such, but is often used in conjunction with insecticides as it delays their breakdown, hence making them active for longer.

A study found that exposure to PBO in children younger than 3 years old had a negative impact on brain development. (2) It is also listed as a Possible Human Carcinogen by the US Environmental Protection Agency. (3)


This is one of the safer ones, so if you must use a chemical product, this is a better choice. It is however, harmful to bees and other sucking insects. There is a chance of toxicity to the human nervous system. An interesting study found transferrable residue on the fur of dogs 4 weeks after treatment. (4)


Pyrethrin containing products are often marketed as “natural” because they come from an extract of Chrysanthemum flowers called Pyrethrum. Yet EPA classifies them as “Suggestive evidence of carcinogenicity but not sufficient to assess human carcinogenic potential.” (1) They also often cause allergic reactions as well as exacerbate asthma. Difficulty breathing, wheezing, nasal discharge are several of the symptoms to look out for. Also very toxic to cats.


Fipronil is a possible human carcinogen. (3) It also blocks the GABA pathway in the human brain, causing over-excitation of the nervous system and possible headaches, seizures and stomach problems.

Worryingly, when exposed to sunlight, Fipronil can turn into another molecule (Fipronil sulfone) that is ten times more potent.

Both molecules have been shown to transfer to children from the dog. (6)

Another paper has shown that both of the above molecules disrupt the endocrine system via oestrogen receptors and the thyroid gland. (7)


Carbaryl is banned in many countries including the EU (not banned in South Africa). It is a potent acetylcholinesterase inhibitor and can cause over-excitation of the nervous system, resulting in nausea, vomiting, tearing eyes. (8) A larger dose can result in seizures, respiratory paralysis and death.

It is classified by EPA as “likely carcinogenic to humans”. (3)

Carbaryl has been shown to have adverse effects on fertility of male rats.

Cedarwood Oil

On the other hand, essential oils such as Cedarwood, have not been as extensively studied. They have however many decades of traditional use with few if any adverse effects.

Asthmatic humans should avoid inhaling Cedarwood oil as it might exacerbate the asthma.

Cedarwood oil is NOT safe for cats.


1. NRDC Tick and Flea Products Directory

2. Horton MK, Rundle A, Camann DE, Boyd Barr D, Rauh VA, Whyatt RM. Impact of prenatal exposure to piperonyl butoxide and permethrin on 36-month neurodevelopment. Pediatrics. 2011;127(3):e699-e706. doi:10.1542/peds.2010-0133

3. Chemicals Evaluated for Carcinogenic Potential Annual Cancer Report 2018 (

4. Craig MS, Gupta RC, Candery TD, Britton DA. Human exposure to imidacloprid from dogs treated with advantage(r). Toxicol Mech Methods. 2005;15(4):287-91. doi: 10.1080/15376520590968842. PMID: 20021094.

5. Stephen W Page, Chapter 10 – Antiparasitic drugs, Editor(s): JILL E MADDISON, STEPHEN W PAGE, DAVID B CHURCH, Small Animal Clinical Pharmacology (Second Edition), W.B. Saunders, 2008, Pages 198-260

6. Cochran RC, Yu L, Krieger RI, Ross JH. Postapplication Fipronil Exposure Following Use on Pets. J Toxicol Environ Health A. 2015;78(19):1217-26. doi: 10.1080/15287394.2015.1076363. Epub 2015 Oct 5. PMID: 26437174.

7. Lu M, Du J, Zhou P, Chen H, Lu C, Zhang Q. Endocrine disrupting potential of fipronil and its metabolite in reporter gene assays. Chemosphere. 2015 Feb;120:246-51. doi: 10.1016/j.chemosphere.2014.07.015. Epub 2014 Aug 9. PMID: 25112704.


9. Fattahi E, Jorsaraei SG, Gardaneh M. The effect of Carbaryl on the pituitary-gonad axis in male rats. Iran J Reprod Med. 2012;10(5):419-424.

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