<qa>  What is the effect of tibolone on the endometrium?In a double-blind, randomized controlled trial, Hammar 16 showed that the bleeding andor spotting episodes in tibolone users in the first months of use were significantly lower than in E2NETA users, and the authors hypothesized that this was due to a higher progestagenic activity of tibolone on the endometrium compared to the combined treatment. Our in vitro studies show indeed that tibolone can exert a progestagenic effect and that the balance between the progestagenic properties and the estrogenic properties of tibolone takes effect through a different set of genes acting in the same genetic network 17, 18. 