i had my nipple pierced, and 3 days after i got it done someone twisted my nipple in school, after which followed infection. i now have a hyper-sensitive nipple, it's very sensitive to touch and just a real pain in the ass.
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I think what the other person is trying to say is that aside from the basic risk factors you run the chance of either hypersensitivity or hyposensitivity. Other than there, not much in the way of risks or negative effects.