The Lab
Specimen No. 0204
AvoidSyntheticsilicone

Cyclotetrasiloxane (D4)

CAS 556-67-2 · D4 · Octamethylcyclotetrasiloxane · Cyclotetrasiloxane

A volatile cyclic silicone that the EU has prohibited outright in cosmetic products (Annex II), primarily on environmental persistence grounds; its rodent reproductive effects appear species-specific rather than human-relevant, but the regulatory prohibition makes it one to avoid on a label.

high confidence
Researched July 7, 2026

Concerns

  • The EU has added D4 to Annex II of the Cosmetics Regulation — the list of substances prohibited in cosmetic products — as entry 1388 (Commission Regulation (EU) 2019/831), so it is banned outright in EU cosmetics, not merely restricted
  • The EU Risk Assessment Committee concluded that D4 meets the REACH criteria for a persistent, bioaccumulative and toxic (PBT) substance and a very persistent, very bioaccumulative (vPvB) substance, and it was separately restricted from wash-off cosmetic products at 0.1 percent or more by weight since 31 January 2020 (EU Regulation 2018/35)
  • Reproductive effects observed in rodent studies arise from a rat-specific delay of the mid-cycle gonadotrophin-releasing-hormone surge that is not relevant to human reproduction, so on this mode-of-action analysis D4 should not be classified as a CMR substance or an endocrine disruptor (Andersen 2022, PMID 34995712)
  • A weight-of-evidence, biochemical and molecular-docking analysis found the estrogenic endocrine-disruptive potential of D4 to be molecularly, biochemically and physiologically implausible (Borgert 2025, PMID 39976757)

Regulatory flags

  • IARC Class: NL
  • EDC Status: Disputed - estrogen-pathway EDC not supported; rodent-specific reproductive MOA
  • Regulatory: EU: PROHIBITED in cosmetics (Annex II #1388, Reg 2019/831); PBT/vPvB SVHC; REACH wash-off limit <0.1% (2020)
  • Evidence Grade: A (regulatory ban) / B (environmental)

Commonly found in

Hair conditioner and anti-frizz serumDeodorantFoundation and primerSkin cream

Research disclaimer: This rating is based on available peer-reviewed research and regulatory assessments at the time of publication. It is not medical advice. Consult a qualified professional for personal health or skin concerns.

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LabelCleared verdict badge: Cyclotetrasiloxane (D4) — Avoid
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