Buying a second-hand menstrual cup
Don't! Believe it or not, this can and does happen. Some women advertise their "lightly-used cups" for sale on a few websites. Considering the fact that menstrual cups are placed in such an intimate part of our bodies, buying a used cup or even sharing cups is a big no no. Don't put yourself at risk of contracting sexually transmitted diseases or infections.
Instead, share the knowledge and spread the word to as many women as you can.
Wearing your menstrual cup during sex
After inserting your menstrual cup for the first time, this thought should never cross your mind. You may have read about ladies using cups to have sex during their period; they aren't talking about menstrual cups. They are referring to a product called Instead SoftCup. As the name implies SoftCups are soft, the rim of the cup is firm while the body of the cup is a soft, thick plastic.
Edit*: As pointed out by one commenter below, I may have been too general in my definition of sexual activity. While wearing a menstrual cup, a couple should never engage in penetrative sex. However, because the menstrual cup is quite effective at keeping menstrual fluid contained, oral sex and other forms of external stimulation are still a possibility as long as all parties involved are open to the idea.
Exercising with your menstrual cup
Do! This is one of the greatest benefits of using a menstrual cup. It 's worn internally, so no sporting or leisure activity is off-limits. And with the ability to hold 3 times as much fluid as pads and tampons, your fun won't be interrupted by the need to change your menstrual product.
Using a menstrual cup if you've had TSS
Don't. If you have ever suffered from toxic shock syndrome (TSS), even if it was only once, you should never use a menstrual cup or any other form of internal menstrual protection. TSS occurs when Staphylococcus bacteria invade the body via an open wound (cut, scrape, etc.) and produce toxins which triggers the immune system into overdrive and eventually results in multi-organ failure. If you've had TSS once, you may be at a higher risk of contracting it in the future.
Menstrual cups provide a safer alternative to tampons and before 2015 they were not linked to TSS. Currently, there has been one confirmed case of a 37 year old woman that contracted TSS 10 days after wearing a Diva cup for the first time. According to the scientists in the report, while the silicone material of the cups prevent microbial growth, the blood collected inside the cup can provide suitable growing conditions for the S. aureus bacteria.
If you experience fever, vomiting, diarrhea, skin rash or other flu symptoms while using a menstrual cup, remove it and go immediately to the closest hospital.
Using a menstrual cup when you have a yeast infection
Menstrual cups are made from medical grade materials that prevent bacterial growth. However, it's always best to err on the side of caution. If you develop a yeast infection a few days before or during your period, stop using the menstrual cup. Boiling your cup may not kill all of the yeast cells and you'll risk re-infecting yourself during your next period.
Some manufacturers recommended that you replace the cup, but let's be realistic. Unless you've gotten 2+ years of use from your cup none of us want to willingly toss them. My personal, unqualified recommendation is to ask your gynecologist his/her opinion. They will be able to guide you in the right direction and suggest a product you can use to effectively clean your menstrual cup.
Using a menstrual cup with an IUD
It's possible, but talk to your gynecologist first. Some doctors, particularly those familiar with menstrual cups, may be more open to the suggestion than others. If given the go ahead, always ensure that you've broken the suction seal before attempting to remove the menstrual cup. Failure to do so can result in you dislodging or removing the IUD.
Using a menstrual cup with an IUD may be easier if you have a high cervix, as it's best to place the cup as low as possible to help keep some distance between the cup and IUD string. You can have your doctor trim the stem if it's too long.
If your cervix is low and normally sits inside the cup during your period, it will be a lot more challenging using a menstrual cup with an IUD in place.