Spironolactone is a potassium-sparing diuretic that works by telling your kidneys to excrete more water, it works on a specific part of the little tubes in your kidneys that reabsorbs potassium (I dunno why they're so ridiculous, you got parts that toss out electrolytes, parts that absorb some of them, parts that throw it out again, all along the same tube lol). Spiro ALSO has a nice amti-androgen effect that we take advantage of off-lable for feminizing HRT, it's mainly used by cis people for the diuretic effect. Just a nice side discovery that when cis men were on it for a while to help manage blood pressure they ended up getting boobs lol.

You have to be mindful of your potassium because your body is retaining more potassium than usual when you're on spiro, the more spiro the more it'll retain. So long as you eat healthy and your otherwise pretty healthy, taking spiro shouldn't do anything harmful - you should still get a lytes panel after getting spiro increased just to make sure your still in the right zone for potassium levels. Potassium is used for a bunch of cell signaling stuff all over your body, one of the big ones is in your heart - too high potassium can lead to heart issues (but it usually has to be really super high before anything noticeable).

https://transfemscience.org/articles/spiro-testosterone/

These findings suggest that spironolactone has inconsistent and limited effects on testosterone levels. Moreover, these data, as well as studies of estradiol alone, indicate that estradiol is mainly responsible for lowered testosterone levels when the combination of estradiol and spironolactone is used for hormone therapy in transfeminine people. Besides testosterone suppression, spironolactone also acts as a direct antagonist of the androgen receptor, and this importantly contributes to its antiandrogenic efficacy as well. However, studies in cisgender women suggest that spironolactone is a relatively weak androgen receptor antagonist, and is likely best-suited for blocking relatively low testosterone levels.