If sore nipples are persistent, it may be because the baby has a thrush infection in his mouth, which has spread to the nipples.
Whenever a mother has sore nipples, she must always check her baby for thrush so that it can be treated before the soreness becomes persistent. A baby with thrush has a white-coated tongue or white lesions that look like milk curds inside the cheeks. If you are not sure, try to wipe them away. If you can remove the white easily, it is milk; if you cannot remove it, then it is probably thrush. You can treat thrush with gentian violet (0.5-1 per cent), or other treatment, suggested by your doctor.