Re how does HRT work - it is a very complex thing - not sure if it's even fully understood yet...Oestrogen is responsible for and affects many functions in our body and there are oestrogen receptors everywhere - also several different types of oestrogen receptor too. It has different effects in different parts of the body where the receptors are (known as target tissues) - or known as tissue specific effects.
The interesting thing about it is that oestrogen levels do rise and fall suddenly - in the menstrual cycle and the effects of this are felt immediately. This is because there are oestrogen receptors on some cell-membranes (so this is one level of operation) - we are all familiar with feeling great aorund ovulation and lousy just before our period. Some women can feel the rise in oestrogen following menstruation or two or three days in - I certainly used to. it is the rapid changes in oestrogen that give rise to PMDD or extreme pms ( progesterone also plays a part).
The other main effects of oestrogen are through the cell nucleus and are called genomic effects - these are the long-term ones eg skin, bones etc. Various proteins are activated and give rise to the changes. It all involves very complex cell biochemistry which only the real specialists who study and write papers about it know about in any depth! I certainly don't - only to know that it works on two levels due to the location and action of oestrogen receptors.
That's probably not the answer you were looking for and Taz's is much more practical!!! As she says- transdermal HRT is preferred for those who have digestive problems or nausea with tabs....
In terms of the Q you were asking CLKD - replacement HRT can also work in both ways - short-term - pretty immediate feelings of well-being, and long-term. Vaso-motor symptoms - seems to work pretty quickly in mosrt women. Most of its benefits are long term - it needs to bind to the receptors in the cell nuclei, and get all the cells busy doing their stuff.....so yes it does need to build up - or rather the effects do....if you see what I mean? Is that too rambly! Sorry if so
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Hurdity x