Can I pray with a tampon or menstrual cup in?

During your period: no prayer at all

While you're menstruating, prayer is lifted from you completely, and you don't make it up later. Wearing a tampon or cup doesn't change that. Even if it stops any blood from showing, you're still on your period. See Do I have to pray while on my period?

Once the bleeding has clearly stopped, you take a ghusl and start praying again. See When can I pray again after my period ends?

Spotting and irregular bleeding

Bleeding that isn't your period (called istihadah) doesn't stop you from praying. A woman named Fatimah bint Abi Hubaysh had this kind of bleeding, and the Prophet ﷺ told her it came from a vein, not menstruation, so she should wash the blood and pray (Sahih al-Bukhari 228). Many women today use a tampon, cup or pad in this situation to keep their clothes clean, and that's fine.

If the bleeding is constant, most scholars say you make a fresh wudu for each prayer after its time begins, then pray, even if a little blood comes out.

Does the tampon or cup break wudu?

Scholars look at whether anything leaves the body:

• Inserting it: doesn't break wudu, but it's easier to put it in first and then make wudu.
• While it's inside: what's inside the body isn't counted as impurity on you. Many Hanafi scholars say wudu holds as long as no blood reaches the outside.
• Removing or emptying it: the Shafi'i and Hanbali schools say anything coming out of the private parts breaks wudu, so you'd renew it.

To be safe, change or empty it first, clean yourself, then make wudu and pray. See What breaks wudu?

Is it allowed to use them at all?

Yes. Nothing in Islam forbids tampons or menstrual cups. They are personal hygiene products, and you can choose what suits your body. Follow the product instructions and your doctor's advice on how long to wear them.