# Coumachlor

**Coumachlor** (CAS 81-82-3) is a first-generation hydroxycoumarin anticoagulant rodenticide, a structural analog of warfarin in which a chlorine atom replaces one hydrogen on the phenyl ring. Its CAS name is 3-[1-(4-chlorophenyl)-3-oxobutyl]-4-hydroxy-2H-1-benzopyran-2-one, and its molecular formula is C\(_{19}\)H\(_{15}\)ClO\(_{4}\) with a molecular weight of 342.77.<sup>[1](https://www.drugfuture.com/chemdata/coumachlor.html)</sup><sup> • </sup><sup>[2](https://www.inchem.org/documents/ehc/ehc/ehc175.htm)</sup> It is a vitamin K antagonist that kills rodents by interfering with blood coagulation.<sup>[3](https://www.medchemexpress.com/coumachlor.html)</sup>

| Key fact | Detail |
|---|---|
| Chemical identity | CAS 81-82-3; C\(_{19}\)H\(_{15}\)ClO\(_{4}\); molecular weight 342.77; CAS name 3-[1-(4-chlorophenyl)-3-oxobutyl]-4-hydroxy-2H-1-benzopyran-2-one<sup>[1](https://www.drugfuture.com/chemdata/coumachlor.html)</sup> |
| Class | First-generation hydroxycoumarin anticoagulant rodenticide, alongside coumafuryl, coumatetralyl, and warfarin<sup>[2](https://www.inchem.org/documents/ehc/ehc/ehc175.htm)</sup><sup> • </sup><sup>[4](https://www.pharmaco.vet.ku.ac.th/pdf_file/Pesticide_20161020.pdf)</sup> |
| Other names | p-Chlorowarfarin, Tomorin, Rival, manufacturers' code G-21133<sup>[5](https://precision.fda.gov/uniisearch/srs/unii/UCD8XZW42P)</sup><sup> • </sup><sup>[1](https://www.drugfuture.com/chemdata/coumachlor.html)</sup> |
| Mechanism | Inhibits hepatic vitamin K1 epoxide reductase, reducing regeneration of active vitamin K and preventing the activation of vitamin K-dependent clotting factors, including prothrombin<sup>[6](https://hero.epa.gov/reference/1360481/)</sup><sup> • </sup><sup>[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC6996659/)</sup> |
| Field performance | In comparative trials on common rats, coumachlor baits at 0.005% killed 2 of 11 and 4 of 11 animals, against near-complete kills with warfarin at the same concentration<sup>[7](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/94C9ACAF284DB31F911F41A635A7662D/S0022172400019975a.pdf/div-class-title-the-comparative-rodenticidal-efficiency-of-five-anti-coagulants-div.pdf)</sup> |
| Regulatory status | Not registered as a plant protection product in Turkey following a final regulatory action banning it; no longer registered for use in California<sup>[8](https://www.pic.int/Portals/5/download.aspx?d=UNEP-FAO-RC-PUB-PicCircular-54.English.pdf)</sup><sup> • </sup><sup>[9](https://www.cdfa.ca.gov/plant/IPC/vertebrates/docs/vpcrac/reports/Assessing_some_potential_environmental_impaxts_from_agricultureal_anticoagulant_uses_copy.pdf)</sup> |

## Chemistry and pharmacology

Coumachlor belongs to the 4-hydroxycoumarin series of anticoagulants, a group derived from coumarin that also includes dicoumarol and warfarin.<sup>[6](https://hero.epa.gov/reference/1360481/)</sup> Like the other hydroxycoumarin rodenticides, it acts as a vitamin K antagonist: these compounds inhibit hepatic vitamin K1 epoxide reductase, the enzyme involved in the regeneration of active vitamin K, preventing activation of vitamin K-dependent clotting factors, including prothrombin, so blood fails to clot and the animal dies of hemorrhage.<sup>[6](https://hero.epa.gov/reference/1360481/)</sup><sup> • </sup><sup>[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC6996659/)</sup> A 1953 study in *Acta Pharmacologica et Toxicologica* by H. Dam and E. Søndergaard compared its anticoagulant action with that of dicoumarol in chicks and rats.<sup>[10](https://pubmed.ncbi.nlm.nih.gov/13079850/)</sup>

The compound is a racemate, listed in chemical registries as the (±)-form of 3-(α-acetonyl-4-chlorobenzyl)-4-hydroxycoumarin, and it appears in the FDA's UNII inventory as an ingredient substance with the UNII code UCD8XZW42P.<sup>[5](https://precision.fda.gov/uniisearch/srs/unii/UCD8XZW42P)</sup>

## Use as a rodenticide

First-generation hydroxycoumarins, coumachlor among them, were introduced as rodenticides in the late 1940s.<sup>[2](https://www.inchem.org/documents/ehc/ehc/ehc175.htm)</sup> A WHO regional seminar document, citing Bentley (1966), lists coumachlor among the anticoagulants favored in Canada (as Rival and Coumachlor), France (with [chlorophacinone](https://www.edgepedia.org/chlorophacinone) and LM 83), Germany (with [coumatetralyl](https://www.edgepedia.org/coumatetralyl) and fumarin), and Malaya.<sup>[11](https://applications.emro.who.int/docs/EM_SEM_PH_EC_ASP_ROD_CTL_12_EN.pdf)</sup>

**Comparative efficacy.** A published trial series on common rats measured mortality after three days of feeding on poisoned bait. Coumachlor performed poorly at low concentrations: at 0.005% bait it killed 2 of 11 rats in one series and 4 of 11 in another, and even at 0.05% it killed 2 of 11 and 6 of 12, with time-to-death stretching to 55–120 hours at the higher concentrations. The authors concluded that the results would appear to contra-indicate the use of coumachlor at 0.05% or less, since warfarin at 0.005% gave a nearly complete kill of rats that had fed exclusively on it for 96 hours.<sup>[7](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/94C9ACAF284DB31F911F41A635A7662D/S0022172400019975a.pdf/div-class-title-the-comparative-rodenticidal-efficiency-of-five-anti-coagulants-div.pdf)</sup>

## Regulatory status

Under the Rotterdam Convention's PIC Circular, Turkey reported a final regulatory action banning coumachlor as a pesticide, summarizing that coumachlor is not registered as a plant protection product in the country.<sup>[8](https://www.pic.int/Portals/5/download.aspx?d=UNEP-FAO-RC-PUB-PicCircular-54.English.pdf)</sup> In the United States, a California Department of Food and [Agriculture](https://www.edgepedia.org/agriculture) report notes that coumachlor is no longer registered for use in California, though the chemical was still tested for in wildlife monitoring.<sup>[9](https://www.cdfa.ca.gov/plant/IPC/vertebrates/docs/vpcrac/reports/Assessing_some_potential_environmental_impaxts_from_agricultureal_anticoagulant_uses_copy.pdf)</sup> In the European Union, coumachlor appears in ECHA's substance inventory (EC number 201-378-1) with regulatory obligations under REACH and related frameworks.<sup>[12](https://chemicalsinourlife.echa.europa.eu/sv/web/guest/substance-information/-/substanceinfo/100.001.254?_disssubsinfo_WAR_disssubsinfoportlet_substanceId=100.001.254)</sup>

## Detection and monitoring

Modern analytical methods detect coumachlor alongside other anticoagulant rodenticides in biological samples. A validated serum method using quadratic calibration with 1/x weighting reported an instrument limit of detection of 0.05 ng/mL and a limit of quantification of 0.3 ng/mL in solvent for coumachlor, with a method limit of quantification of 2.5 ng/mL in serum.<sup>[13](https://academic.oup.com/view-large/406408876)</sup> An interlaboratory comparison of liver analysis by ultra-performance liquid chromatography with mass spectrometry measured coumachlor recoveries of 92 to 104 percent at spike levels from 130 to 2,000 ng/mL.<sup>[14](https://journals.sagepub.com/doi/10.1177/10406387231178558)</sup> A 2012 HPLC method with fluorimetric detection validated coumachlor among eight anticoagulant rodenticides and reported chromatograms of a cat liver and a bait that tested positive for coumachlor.<sup>[15](https://journals.sagepub.com/doi/full/10.1177/1040638711433354)</sup>

In food monitoring, the [European Food Safety Authority](https://www.edgepedia.org/european-food-safety-authority)'s 2017 scientific report on the 2015 EU coordinated program recorded 3,970 analyses for coumachlor in one country with zero quantifications above the limit of quantification, a quantification rate of 0.00 percent.<sup>[16](https://www.gastronomiaycia.com/wp-content/uploads/2017/04/Authority-2017-EFSA_Journal.pdf)</sup> A University of California study similarly reported that warfarin, chlorophacinone, and coumachlor were rarely detected in wildlife samples.<sup>[17](https://escholarship.org/content/qt7xf8b9kp/qt7xf8b9kp_noSplash_528a84be46174693b5cdba259c834397.pdf?t=nd83yc)</sup>

## References

1. [Coumachlor, DrugFuture chemical data sheet](https://www.drugfuture.com/chemdata/coumachlor.html)
2. [Anticoagulant rodenticides, Environmental Health Criteria 175, WHO/IPCS (1995)](https://www.inchem.org/documents/ehc/ehc/ehc175.htm)
3. [Coumachlor, MedChemExpress](https://www.medchemexpress.com/coumachlor.html)
4. [Pesticide handbook, Faculty of Veterinary Science, Chulalongkorn University (Thai)](https://www.pharmaco.vet.ku.ac.th/pdf_file/Pesticide_20161020.pdf)
5. [COUMACHLOR, FDA UNII record UCD8XZW42P](https://precision.fda.gov/uniisearch/srs/unii/UCD8XZW42P)
6. [Peoples SA (1970). The pharmacology of rodenticides, Vertebrate Pest Conference](https://hero.epa.gov/reference/1360481/)
7. [The comparative rodenticidal efficiency of five anti-coagulants, Cambridge Core](https://www.cambridge.org/core/services/aop-cambridge-core/content/view/94C9ACAF284DB31F911F41A635A7662D/S0022172400019975a.pdf/div-class-title-the-comparative-rodenticidal-efficiency-of-five-anti-coagulants-div.pdf)
8. [PIC Circular LIV (54), December 2021, Rotterdam Convention](https://www.pic.int/Portals/5/download.aspx?d=UNEP-FAO-RC-PUB-PicCircular-54.English.pdf)
9. [Assessing some potential environmental impacts from agricultural anticoagulant uses, California Department of Food and Agriculture](https://www.cdfa.ca.gov/plant/IPC/vertebrates/docs/vpcrac/reports/Assessing_some_potential_environmental_impaxts_from_agricultureal_anticoagulant_uses_copy.pdf)
10. [Dam H, Søndergaard E (1953). Comparison of the anticoagulant action of coumachlor and dicoumarol in chicks and rats, Acta Pharmacol Toxicol 9(2):137–47](https://pubmed.ncbi.nlm.nih.gov/13079850/)
11. [WHO EMRO seminar document EM/SEM.PH.EC.ASF.ROD.CTL/12](https://applications.emro.who.int/docs/EM_SEM_PH_EC_ASP_ROD_CTL_12_EN.pdf)
12. [Coumachlor substance infocard, ECHA](https://chemicalsinourlife.echa.europa.eu/sv/web/guest/substance-information/-/substanceinfo/100.001.254?_disssubsinfo_WAR_disssubsinfoportlet_substanceId=100.001.254)
13. [Table I, method originating laboratory in-house validation data, Oxford Academic](https://academic.oup.com/view-large/406408876)
14. [Chen Y et al. (2023). Validation and interlaboratory comparison of anticoagulant rodenticide analysis in animal livers, Journal of Veterinary Diagnostic Investigation](https://journals.sagepub.com/doi/10.1177/10406387231178558)
15. [Armentano A et al. (2012). Validation and application of multi-residue analysis of eight anticoagulant rodenticides, Journal of Veterinary Diagnostic Investigation](https://journals.sagepub.com/doi/full/10.1177/1040638711433354)
16. [EFSA Journal (2017) scientific report on the 2015 EU coordinated pesticide monitoring programme](https://www.gastronomiaycia.com/wp-content/uploads/2017/04/Authority-2017-EFSA_Journal.pdf)
17. [University of California report on anticoagulant rodenticide detections](https://escholarship.org/content/qt7xf8b9kp/qt7xf8b9kp_noSplash_528a84be46174693b5cdba259c834397.pdf?t=nd83yc)
18. [pmc.ncbi.nlm.nih.gov](https://pmc.ncbi.nlm.nih.gov/articles/PMC6996659/)

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*Topic: Encyclopedia › Life and health › Applied biology and nonhuman health › Plant disease and plant protection › Pesticides › Rodenticides*

*Initially written Oct 21, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license