Progesterone reduces the number of estrogen receptors. This is well established. What this means is tissues become LESS SENSITIVE to estrogen as a result.
Progesterone also increases the enzyme which converts estradiol (the strongest of the estrogens found in the human body) to estrone (the second strongest, but weaker in about 12x than estradiol). What this means is you have less of the strongest estrogen circulating in your tissues.
Both of the above mechanisms are ways in which progesterone exerts anti-estrogenic effects. But, progesterone also, and this is unequivocally established by studies, stimulates milk glands (lobules, alveoli) while estrogen stimulates ducts and ductal branching, those things that help milk circulate all the way to the nipples. They work together to help prepare for lactation.
A few studies have found that estrogen alone increases breast growth but the addition of progesterone limits this growth or stops it. Others have noted that greatest breast growth during a woman's menstrual cycle is during the luteal phase, when progesterone and estrogen levels are high.
Now, as far as feeling good or bad on progesterone, more studies have found progesterone to have an adverse effect on physical and psychological symptoms than not BUT the ways in which this conclusion was reached can be questioned. For instance, in those postmenopausal women given estrogen, was the progesterone dose much too high relative to that of estrogen, typically a weak dose. Very well possible. Other studies looked at levels of estrogen in women relative to progesterone and often found levels of progesterone to be lower or estradiol levels to be higher in those women who experienced PMS. This suggests an imbalance in estrogen relative to progesterone. In other words, too much estrogen relative to progesterone. But, then again, studies have tried giving women with PMS more progesterone during that time with very few positive results. The only somewhat encouraging results were with oral progesterone and this could well be due to the higher levels of sedative neurosteroids resulting from oral intake that make some of us giddy and "high". On the other hand, reduce the concentration of those neurosteroids and some women feel better.
Results seem to be all over the place and I'm still researching this matter, trying to find some pattern somewhere. In the meantime, draw your own conclusions, give it a try with your doctor's consent and see what happens with YOU. You like it, health is good, boobs are growing, then all the better for you. Otherwise, dump it and continue with E. That simple. Trial and error.
I've taken progesterone for more than 3 years and still to this day, remain uncertain as to whether it's needed in my case.
