Hi Flamingo
In fact there has been a lot of work done on the pharmacokinetics of Utrogestan (ie what happens to it in the body and the time course of oral vs vaginal) and is reported in various specialist papers.
Of course all studies will vary to some extrent as the trials won't be exactly the same but the answers or issues to think about are more complex.
It isn't a question of "some people claim" in relation to absorption. There is evidence based information.
The main points (from memory!) are:
1 When the same dose of Utrogestan is administered orally and vaginally MORE is delivered straight to the uterus with vaginal administration, because it avoids what is known as the liver first pass effect (I think that's the right term...) whereby oral drugs need to be digested and then metabolised through the liver. Therefore for the purposes of protecting the uterus vaginal delivery is optimal though oral intake is fine for many women.
2 Taken orally, there are many more and larger quantities of metabolic by-products than when taken vaginally, and it is these by-products that lead to some of the more undesirable side effects some women experience with oral intake, hence the French product info advises vaginal use AT THE SAME DOSE if side effects are experienced with oral use.
3 The large paper on pharmacokinetics I read, in the study quoted there, there was indeed GREATER systemic absoprtion (as well as more prog being delivered to the uterus) from vaginal vs oral delivery at the same dose. This is not in fact surprising since much (?some) of the oral dose is lost to digestion/liver metabolism so the amount getting into the bloodstream is much less, and indeed has to be delivered to the uterus from the blood system. This is why oral Utrogestan may sometimes not be so effective at protecting the uterus, especially at higher oestrogen doses which I don't think have been researched so well (compared with low/medium oestrogen doses).
As for benefits on skin and hair - as far as I know these are not benefits of progesterone but correct me if I'm wrong or if there is recent research (rather than journalist articles or lay information on some general websites) on this - it is oestrogen especially that has positive effects on our skin and hair and lack of it, the opposite. I would say more to the point is which method of progesterone delivery will have the least bad effect on skin and hair (though maybe more of the negative effects are from the synthetic progestogens).
If you can't take it orally - then take it vaginally - but the bottom line is, you need enough to protect your uterus, and you need to have a sufficient dose of oestrogen to optimise other health benefits (though I realise that is a vague statement!)
Hope this helps and all the best with your decision!
Hurdity x