I know plenty of people in this comm successfully use unscented castile soap on their cups (I use scented castile, myself--it's just what I had on hand). I think as long as whatever soap you're using isn't antibacterial and you rinse your cup very thoroughly after using soap on it to make sure there isn't any residue, then most soaps are ok.

I would think an all natural soap would be better than a not so natural soap for a cup.

'Natural' is such a relative term these days, you can pretty much put it on anything. And chemicals are used for a reason, quite often they do the job more efficiently. For example, using milton steralising tablets might be better than using a heavily scented soap. Though YMMV, and it depends on your comfort level.

To the OP: the soap would probably 'work' as in make it clean, but make sure you remove any residue and scent it might leave behind. you probably don't need to use soap every single time you wash it, either. Good luck!

I think that ANY soap is fine, as long as you rinse all the residue away. The residue could be irritating to your (ahem) delicate membranes and some ingredients (especially glycerins) can make yeast infections bloom. Some say that anti-bacterial soap should be avoided because it clobbers the "good bacteria" as well as the bad -- but as long as you rinse it very, very well, I personally don't think it matters.

If you're just going to pop it right back in, then just a quick rinse with water is more than enough.

Some gals are fanatical about boiling or using chemical methods. Not me. "As clean as cutlery" is clean enough.

I always had issues with any soap irritating my *lady parts* even just on the outside. but i have been using the Diva Wash that I got with my Diva cup and it hasn't irritated me whatso ever. i have used it to clean my cup and when I feel I need to also my *lady parts* and have not gotten any reaction to it at all. but good luck!!