What are some effects that pacifiers have on teething?

Possible contriction. With pacifiers, one of the main long term side effects is constriction of the upper jaw. If the pacifier is used beyond a 'normal pacifier age', then the constant sucking action may prevent the maxilla(upper jaw) from growing properly which will cause the upper teeth to be malpositioned and may cause problems with the patients bite and appearance later in life.
Positive ones. You can freeze a pacifier to numb the gums, they like chewing, it helps, plus a pacifier 'till age 3-4 yrs (nuk) helps the jaws grow, and lowers the chance of needing braces.