Why Does Progesterone Have a Cancer Warning on the Bottle?

If you’ve seen the California Proposition 65 cancer warning on natural progesterone, you’re not alone in wondering why it’s there. Here’s what you need to know

1. California doesn’t separate progesterone from progestins

Progesterone is the natural hormone your body makes. Doctors even give it to pregnant women to prevent miscarriage and protect pregnancy [1].
Progestins are synthetic, drug-like versions of progesterone, used in most birth control and hormone replacement drugs. They are known to cause serious complications like blood clots and miscarriages [2].

👉 California law lumps them together. That’s why natural, bioidentical progesterone carries the same warning—even though the science shows they are very different.

2. How doctors actually use natural progesterone

Natural bioidentical progesterone (micronized progesterone, identical to what your ovaries make) has been used safely for decades in critical medical situations:

Preventing miscarriage in early pregnancy [1]
Reducing the risk of preeclampsia and gestational diabetes [3]
Lessening morning sickness [3]
Supporting brain healing after traumatic brain injury [4]
Helping manage catamenial seizures (hormone-related seizures) [4]

If natural progesterone were truly cancer-causing, doctors would never use it in these situations.

3. Why the warning exists

California’s Proposition 65 law requires a warning label on any substance ever linked to cancer, even if the studies are based on:

Synthetic versions (progestins),
Extremely high doses, or
Animal data that don’t reflect real-world human use.
Synthetic progestins: have been implicated in blood clots, fibroids, high blood pressure, heart disease, and even triggering catamenial epilepsy [2].
Natural progesterone: has documented protective effects on the brain, heart, lungs, and even certain cancers [4].

Because the law doesn’t distinguish between them, all progesterone products—natural or synthetic—carry the same warning.

4. The bottom line

The Prop 65 warning doesn’t mean your natural progesterone is unsafe. It simply means California applies a broad legal label to both synthetic progestins and natural progesterone.

👉 Think of it this way:
It’s like California requiring all “oils” to carry a heart disease warning—without telling you whether it’s olive oil or trans fats.

✅ Key takeaway:
Natural bioidentical progesterone has a decades-long track record of safe use and is even prescribed to protect pregnant women and their babies. The Prop 65 warning is a legal formality in California, not a reflection of the actual science.

-Kitty Martone

References

1. [Progestins are never given to pregnant women due to risk of miscarriage](https://pubmed.ncbi.nlm.nih.gov/17350963/)

2. [Risks of synthetic progestins in birth control and HRT](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2841566/)

3. [Natural progesterone supplementation reduces miscarriage, preeclampsia, and gestational complications](https://pubmed.ncbi.nlm.nih.gov/19032071/)

4. [Full study: Molecular and receptor differences between progesterone and progestins](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3242720/)

Criticisms of California’s Proposition 65

Alarm Fatigue & Over-Warning – Prop 65 labels are everywhere, often applied “just in case,” which dilutes their meaning and causes consumers to ignore them (Wikipedia).
Unnecessary Burden & Lawsuit Risk – The law enables costly lawsuits against businesses, even when chemical levels are far below harmful thresholds (J.F. Novak Law blog).
Lack of Scientific Basis – Some warnings are required despite weak or disputed evidence. Courts have even ruled certain Prop 65 warnings unconstitutional for being misleading (Greenberg Glusker legal article).
Mixed Effectiveness – While some companies reformulate products to avoid Prop 65 chemicals, critics argue the law often misleads more than it informs (Silent Spring Institute).

-Kitty Martone

Disclaimer: Please note that the information shared in these articles is for educational purposes only and should not be considered as medical advice. Always consult with a qualified healthcare professional for personalized guidance regarding your health concerns.