As Evelyn stated, spiro is commonly used with estrogen in the US for trans HRT. There’s some anecdotal evidence it inhibits breast development. It also has significant side effects. I did a ton of research before going on HRT and decided I would avoid spiro if at all possible. My doc isn’t a fan and that’s one of the reasons I chose her. I’ve been on EV monotherapy from the start. E and T levels have been easy to maintain at therapeutic (female) levels.