02/09/2026
The Women’s Health Initiative combined-hormone trial studied more than 16,000 postmenopausal women and used:
Conjugated equine oestrogen (Premarin) + medroxyprogesterone acetate (MPA) — a synthetic progestin. Notably, the increased breast cancer risk was not observed with oestrogen-only therapy (ET), which instead showed a potential protective effect. It’s worth noting this was administered orally, however — which is not something I would choose to do!
The E3N study found a higher breast cancer risk with oestrogen combined with synthetic progestogens, compared to oestrogen combined with micronised progesterone. A subsequent meta-analysis found a similar pattern.
But here’s where it gets interesting…
PROGESTERONE AND SYNTHETIC PROGESTINS ARE NOT THE SAME THING.
Micronised progesterone (Prometrium) is chemically identical to the progesterone our bodies produce naturally.
Synthetic progestins, on the other hand, are different compounds engineered to mimic progesterone-like effects.
Research suggests that breast cancer risk may differ depending on which progestogen (the umbrella term) is used alongside oestrogen (see post regarding forms).
One more thing worth mentioning: the Mirena.
The Mirena IUD contains levonorgestrel, which is also a synthetic progestin. Unlike oral progestins, it delivers levonorgestrel primarily within the uterus, resulting in lower systemic exposure than oral administration.The Mirena is also sometimes used alongside oestrogen MHT to provide endometrial protection.
So, when discussing “progesterone” in the context of MHT, contraception, or the Mirena, it’s essential to know exactly which progestogen is being used — and how it’s being delivered.
This isn’t medical advice.
That said, I do think it’s worth being informed about the hormones you’re using — and understanding that progesterone and synthetic progestins are not the same thing.
Because when it comes to hormones, the details matter.