My massage therapist told me her recipe:
1 cup baking soda (absorbs toxins so you don't reabsorb)
1 cup epson salts (to draw out toxins)
1 cup Sea Salt (preferably dead sea but its expensive)
Don't dump right in bath. Instead put in container and pour boiling water on it, stir till dissolved, and pour that in bath. The boiling water maximizes the effectiveness (or releases effectiveness).
Hydrogen peroxide bath is good even if low quantity. I did one and for the 1st time in years have not had cramps with period. I did one last night with my liver cleanse, too, but only had about 1/6th the recommended amount. (3 bottles of 3%). Get food grade at health food store, it's better, fewer contaminants.