Brief Answer:
The cause is usually benign-more info needed

Detailed Answer:
Hello,
I'll try to answer your question by presenting possible causes of your problem. More information would be needed for a more complete answer.
Most cases of itching are caused by allergic reactions which cause inflammation to the skin (eczema). Several irritants can be the cause as well.
For example if you breastfeed a baby then your nipple and areola can get irritated enough to cause this inflammation.
Try to remember if you're exposed to chemical substances like a new soap, perfume etc that you haven't used before.
Finally I should mention that rare cases of cancer can present with itching. Paget's disease for example causes an eczema like condition.
If both breasts are itchy then the cancer scenario is even more unlikely.
Although chances are you're having just an eczema and you shouldn't worry about that, I suggest you visit your doctor for visual inspection and palpation of the breasts.
Should you need clarifications on the subject, feel free to ask again. You may provide more information if you want, like :
- is there any visible lesion?
- is the nipple, areola, surrounding skin involved (which if any) ?
- are any other areas of your body affected?
- do you have a history of allergy?
- did you take any medication?
- are you breastfeeding?

Kind Regards!

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