pyrethrin
noun
py·re·thrin
pī-ˈrē-thrən 
-ˈre-
-ˈre-
: either of two oily liquid esters C21H28O3 and C22H28O5 having insecticidal properties and occurring especially in the flowers of pyrethrum
Examples of pyrethrin in a Sentence
Recent Examples on the Web
Botanical alternatives such as neem and pyrethrins in canola oil provide three to four days of deterrence.
—
Arricca Elin Sansone, Southern Living, 7 May 2025
The residual sprays can protect your house for up to 45 days, while the pyrethrins, which are short-contact sprays, will irritate the bugs and send them into the residual spray.
—Nafeesah Allen, Better Homes & Gardens, 4 Jan. 2025
Complicating matters further is the fact that lice appear to have evolved some resistance to pyrethrin and permethrin, the active ingredients in many over-the-counter lice shampoos.
—Anna North, Vox, 7 Dec. 2018
Look for products containing ingredients such as pyrethrin, bifenthrin, or carbaryl.
—Nafeesah Allen, Better Homes & Gardens, 26 July 2024
According to the county, the pesticide is an ultra-low volume spray of pyrethrin or pyrethroids, which is approved by the Environmental Protection Agency.
—Nollyanne Delacruz, The Mercury News, 26 July 2024
Also, research shows that lice have built up resistance to many popular over-the-counter lice products that contain permethrin and pyrethrin, which are insecticides.
—Lindsey Bever, Washington Post, 28 June 2024
Look for shampoos that contain ingredients like pyrethrin or permethrin, which are known to be effective against fleas at all stages of their life cycle.
—Amber Smith, Discover Magazine, 8 Nov. 2023
Similar to permethrin lotion, pyrethrins don't kill unhatched eggs, so a second treatment is usually recommended 9 days after the first.
—USA TODAY, 16 Mar. 2024
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Arricca Elin Sansone, Southern Living, 7 May 2025
The residual sprays can protect your house for up to 45 days, while the pyrethrins, which are short-contact sprays, will irritate the bugs and send them into the residual spray.
—Nafeesah Allen, Better Homes & Gardens, 4 Jan. 2025
Complicating matters further is the fact that lice appear to have evolved some resistance to pyrethrin and permethrin, the active ingredients in many over-the-counter lice shampoos.
—Anna North, Vox, 7 Dec. 2018
Look for products containing ingredients such as pyrethrin, bifenthrin, or carbaryl.
—Nafeesah Allen, Better Homes & Gardens, 26 July 2024
According to the county, the pesticide is an ultra-low volume spray of pyrethrin or pyrethroids, which is approved by the Environmental Protection Agency.
—Nollyanne Delacruz, The Mercury News, 26 July 2024
Also, research shows that lice have built up resistance to many popular over-the-counter lice products that contain permethrin and pyrethrin, which are insecticides.
—Lindsey Bever, Washington Post, 28 June 2024
Look for shampoos that contain ingredients like pyrethrin or permethrin, which are known to be effective against fleas at all stages of their life cycle.
—Amber Smith, Discover Magazine, 8 Nov. 2023
Similar to permethrin lotion, pyrethrins don't kill unhatched eggs, so a second treatment is usually recommended 9 days after the first.
—USA TODAY, 16 Mar. 2024
Cite this Entry
“Pyrethrin.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/pyrethrin. Accessed 1 Jul. 2025.
Copy CitationMedical Definition
pyrethrin
noun
py·re·thrin
pī-ˈrē-thrən
-ˈreth-rən
-ˈreth-rən
: either of two oily liquid esters C21H28O3 and C22H28O5 that have insecticidal properties and are the active components of pyrethrum
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