If your baby grows comfortable with one certain nipple, he or she may object if you change to a new one. Rotate the use of 2 or more nipples for about 2 months before replacing them. To prevent the nipples from becoming worn out, replace them earlier if necessary, without regard to how many times they have been used. (Regarding isoprene rubber nipples previously sold, those can be used for three to four weeks when two or more nipples alternate.) The nipples may be worn by biting. Special attention is needed for teething child.