Peroxide from any drug store is safe to use directly on a cup. However, it’s not necessary to use it directly. A mix of peroxide and water will be sufficient for an over night soak for a stained cup. The ratio doesn’t have to be measured just as long as you have some peroxide in the mix. Peroxide will turn into water in a short time if left exposed, so over night will be plenty of time for it to work it’s magic.
I personally like to dilute my peroxide with some water because it saves me some money 🙂
Yes. After numerous uses some menstrual cups do wear, tear and accumulate stains. You can try cleaning it and scrubbing it after sterilization to remove the stains or lighten them, however never bleach, use abrasive or toxic cleaners, or other materials to de-stain your menstrual cup as it could permanently damage it.