EFSA: cypermethrin — a neuronal activity study on rat cortical slices allows only a qualitative comparison of the eight isomers; CYP22 and CYP23 are the most active, the (1S cis αR) isomer shows limited potency, but relative potency factors are still not available
On 6 November 2026 EFSA published the conclusions of the peer review of confirmatory data for the insecticide cypermethrin concerning the relative toxicity of the individual isomers. The submitted study is suitable for a qualitative but not a quantitative comparison — so the uncertainty about the isomer composition of residues and the consumer risk assessment left unfinalised in 2018 remain in place.
- The approval of cypermethrin was renewed by Implementing Regulation (EU) 2021/2049 with effect from 1 February 2022, subject to confirmatory data, among others on the relative toxicity of the individual isomers, in particular the enantiomer (1S cis αR) — due by 15 December 2023. The data were submitted on 14 December 2023 by SCC GmbH on behalf of the Cypermethrin Working Group, the assessment was prepared by Belgium, experts discussed it at a meeting on 22–25 June 2026, and the conclusions were approved on 7 October 2026.
- The cypermethrin molecule has three asymmetric carbon atoms, giving four pairs of enantiomers (eight stereoisomers); some of their mixtures have their own ISO common names: alpha-, beta-, theta- and zeta-cypermethrin.
- The applicant submitted a multi-electrode array (MEA) study with rat cortical slices: spontaneous neuronal firing activity was recorded after exposure to each of the eight stereoisomers and to reference items (technical cypermethrin and technical deltamethrin). The EFSA Working Group on Neurotoxicity considered this measurement a suitable proxy for neurotoxicity, consistent with the established mode of action of pyrethroids.
- Qualitative result: the enantiomers CYP22 (1R cis αS) and CYP23 (1R trans αS) are the most active, while the others — including CYP26 (1S cis αR) — show limited potency. However, the data in the study report do not allow a quantitative comparison or the derivation of enantiomer-specific relative potency factors; this would require the raw data and a re-analysis of the study.
- What matters for residue laboratories is that EFSA maintains the uncertainty about the isomer composition of residues in both plant and animal commodities. The relative potency factor of 4 used previously reflected broader uncertainties, and the consumer risk assessment remains unfinalised; the opinion does not concern analytical methods or maximum residue levels.
- Representative uses in the assessment: spraying of cereals for grain and potatoes in southern Europe and of oilseed rape in the central and northern zones, with an emulsifiable concentrate (500 g/l). The endocrine disrupting properties are to be considered by the ECHA Risk Assessment Committee in the harmonised classification.