Pet Safety / Compounds / Mestranol

Is Mestranol safe for dogs and cats?

Moderate risk for pets

Not used therapeutically. Estrogen toxicity risk if ingested (bone marrow suppression, pancytopenia).

What is mestranol?

The IUPAC name is (8R,9S,13S,14S,17R)-17-ethynyl-3-methoxy-13-methyl-7,8,9,11,12,14,15,16-octahydro-6H-cyclopenta[a]phenanthren-17-ol.

Also known as: Norquen, Ovastol, Menophase, EE3ME.

IUPAC name
(8R,9S,13S,14S,17R)-17-ethynyl-3-methoxy-13-methyl-7,8,9,11,12,14,15,16-octahydro-6H-cyclopenta[a]phenanthren-17-ol
CAS number
72-33-3
Molecular formula
C21H26O2
Molecular weight
310.43 g/mol
SMILES
C#C[C@]1(OC)CC[C@@H]2[C@@H]3CCc4cc(OC)ccc4[C@@H]3CC[C@@]21C
PubChem CID
6291

Risk for dogs

Moderate risk

Not used therapeutically. Estrogen toxicity risk if ingested (bone marrow suppression, pancytopenia).

Regulatory consensus

3 regulatory and scientific bodies have classified Mestranol. The classifications differ — that's the data.

AgencyYearClassificationNotes
IARC2012Group 1 — Carcinogenic to humans (combined estrogen-progestogen oral contraceptives)
FDA1960Approved (legacy formulations); largely superseded
EU2015Watch List candidate (Water Framework Directive) via EE2 class

Regulators apply different standards of evidence — animal-data weighting, exposure-pattern assumptions, epidemiological power thresholds — which is why two scientific bodies can review the same data and reach different conclusions. The disagreement is the data.

Where pets encounter mestranol

  • Pharmaceutical
  • Wastewater

Safer alternatives

Lower-risk approaches that achieve a similar outcome to Mestranol:

  • Ethinyl estradiol (lower dose formulations)
    Trade-offs: Alternative approach; specific tradeoffs depend on application context, scale, and regulatory requirements. Full hazard assessment of alternative recommended before adoption to avoid regrettable substitution.
    Relative cost: 1.2-2×
  • Estetrol (E4, next-generation estrogen with reduced thrombotic risk)
    Trade-offs: Improved safety/efficacy profile based on structure-activity optimization; may lack long-term safety data compared to established drugs; patent protection means higher cost until generic entry.
    Relative cost: 1.2-2×

Frequently asked questions

Is mestranol safe for pets?

Not used therapeutically. Estrogen toxicity risk if ingested (bone marrow suppression, pancytopenia).

Why do regulators disagree about mestranol?

Mestranol has been classified by 3 agencies including IARC, FDA, EU, with differing conclusions. Regulators apply different standards of evidence (animal data weighting, exposure-pattern assumptions, epidemiological power thresholds), which is why two scientific bodies can review the same data and reach different conclusions. See the regulatory consensus table on this page for the full picture.

See Mestranol in the pets app

Look up products containing mestranol, compare to alternatives, and explore the full data record.

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Sources (1)
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Reference data, not professional advice. Aggregates publicly available regulatory and scientific data; not a substitute for veterinary, medical, legal, or regulatory advice. Why we built ALETHEIA →