If your menstrual cup has a bad smell, it can be caused be one of these reasons: The cup has not been correctly washed you have used the cup for more time than what is suggested you washed the cup with hot water. Hot water tends to fix stains and smells, for this reason we suggest you to wash the cup with cold water and intimate care soap and to boil it in water only after this operation. if the bad smell does not pass, you can use a sterilizing liquid like Milton solution. After having followed the producer instructions, wash thoroughly the cup with water and soap and then boil it for 3-5 minutes so to remove any residual of the sterilizing solution. if none of these options works, it is the moment to change your menstrual cup. Sometimes old menstrual cups can give this kind of problem.