As a result, the creams aren't standardized, and there is no guarantee of the amount of progesterone in a cream. Nor is there much information about a cream's effectiveness, because the pharmacies aren't required to conduct clinical trials to test the product. This is because the drug has to be absorbed through the skin into the blood. The amount that gets into circulation for distribution to other parts of the body depends on skin thickness and temperature, and therefore varies markedly from individual to individual, making it hard to predict the effect it will have. Third, the scientific evidence for any of the uses of progesterone cream is anything but solid.
Some laboratory studies indicate that progesterone stimulates the growth of breast tissue.
One theory is that intermittent doses may stimulate growth, and constant doses may suppress it 1 . While researchers try to determine how progesterone works on breast cells, epidemiologists are looking at the relationship of progesterone use and breast cancer. Most observational studies have indicated that the addition of a progestin to hormone replacement therapy increases the risk of breast cancer 2,3,4,5 . While progestins are not natural progesterone, they are synthetic cousins that have many of the same effects. In concert, the studies hardly constitute a ringing endorsement for progesterone as a breast cancer preventive.