Great question Thekla
I wouldn’t take them together. I try to take my estriol and progesterone at least 4 hours apart. I use progesterone cream in the morning and late evening. My 8 mg estriol is compounded with 100 mg. progesterone which I take late afternoon but obviously “together”. LOL
The reason I try to space the timing is based primarily on this abstract:Progesterone reverses 17beta-estradiol mediated neuroprotection and BDNF induction in cultured hippocampal neurons
Our results indicate that E2 neuroprotective effects are mediated through the BDNF pathway and that, under certain conditions, P4 antagonizes the protective effect of estrogen.
Clearly, that research isn't about estriol per se and hasn’t even reached mice yet so it’s very early but it gave me a heads up to try and separate the time when I took them. I also checked out (years ago) the half-life of estriol in serum and it was very short—as I recall about 2 hours. It’s interesting because in the latest clinical trial of estriol the 8 mg will be taken in 4 2mg doses throughout the day. Estriol Treatment in MS: Effect on Cognition
I also want to emphasize that MPA which is used in HRT in the US has some risks: Use of MPA for hormone therapy in post menopausal women: Is it safe?
Research into mechanisms by which MPA might cause adverse effects in these areas, combined with the available clinical evidence, suggests a small increase in relative risk for breast cancer and stroke, and a decline in cognitive function, in older women using MPA with an estrogen for postmenopausal HT.
And, there’s this: Differential effects of synthetic progestagens on neuron survival and estrogen neuroprotection in cultured neurons.
We found that medroxyprogesterone acetate decreased both ERα and ERβ expression and blocked E2-mediated neuroprotection and BDNF expression.
All to say I’d definitely stick with bioidentical progesterone. For me, it did take several months to see the benefits of progesterone. All the best to you.