I found this below on the BBC website:
Dummies are quite controversial - some people always use them and find they are a great way to help soothe their child, while others can't stand them and warn about possible risks to the teeth, speech and hearing.
In most cases, children who use dummies don't come to much long term harm. This is probably because they don't actually spend much time with the dummy in their mouth ? using them only intermittently or as a pacifier until they fall asleep at night (when the dummy drops out of their mouth). But there are some risks that you need to be aware of, especially if your child is particularly reliant on their dummy and uses it a lot during the day.
Damage to teeth or jaw?
The biggest concern is that using a dummy (and, it should be said, the 'natural' alternative of thumb-sucking) might damage the teeth or distort the way they grow so that they are left with teeth which don't bite together. But some experts believe that this isn't important while the child has its first set of teeth, or milk teeth, and as long as the child stops using the dummy or sucking their thumb by about 3 to 4 years of age (when adult teeth begin to appear) there is unlikely to be a major problem.
According to the British Dental Association, sucking a dummy is preferable to sucking thumbs, which is more likely to push the teeth out of alignment. In addition, stopping the use of a dummy is easier than getting a child to stop sucking its thumb. Nearly half of thumb-suckers still have the habit at the age of 9 when their adult teeth are coming through (by which time dummies have long been abandoned.) Of course, some dummy users may become thumb suckers but this doesn't happen very often if the child hasn't sucked their thumb from an early age.
Another area of concern with dummies is that they are undoubtedly linked to severe tooth decay when people sweeten them by dipping them in honey or filling them (or bottles) with sugary drinks. Conversely, some people believe that unsweetened dummies may reduce tooth decay by increasing the flow of saliva.
Speech and hearing problems
There is some evidence that the use of a dummy may increase the risk of problems with speech development. With a dummy in their mouth babies may be less inclined or able to babble and practice different shapes with their mouth, and are slow to develop the oral motor skills needed for speech. Research has shown that small children talk more often and more clearly when they aren't brought up with a dummy.
The use of a dummy, if frequent and prolonged, has also been linked to a 33% increase in infections of the middle ear (otitis media) which may interfere with hearing (and thus speech development). However, dummy use is only one of many factors affecting the risk of otitis media and its exact significance is not clear.
Hygiene arguments
Another argument against dummies is hygiene. Most dummies hit the floor dozens of times a day and few parents wash them each time. Some argue that a bit of dirt helps to build the immune system up (research suggests this may be true) but much depends on the sort of dirt the dummy is gathering! For example, if the dummy is dropped in the garden, especially in a household with pets, there is a real risk of infections such as toxoplasmosis.
Risk of stopping breast feeding
Some babies particularly enjoy sucking - it doesn't mean they are anxious or unhappy, just that it is a nice comforting habit.
Others are irritated or confused by putting something other a real nipple in their mouth. The feel and action of a dummy is very different to that of a soft flexible nipple which provides nice warm milk. Babies who use a dummy may spend less time at their mother's breast, reducing the milk supply and causing feeding problems. Research suggests that weaning may occur earlier than it might if dummies were not used. So it is best not to offer a dummy to a baby who is still being breastfed, or at least who has not yet fully mastered the art of breast feeding, especially if they are having problems learning how to latch on or suck correctly.
Protection from Sudden Infant Death
In favour of the use of dummies, there have been three studies in recent years which suggest that they may have a protective effect against Sudden Infant Death Syndrome (SIDS or Cot Death) . Why SIDS occurs is not really understood ? and how a dummy could protect a child is even less clear, but the research results are strong enough to make some doctors hesitant about firmly discouraging the habit.
Simple rules
If you're going to allow a dummy:
Try to limit the amount of time each day that your child is allowed a dummy (preferably just for soothing during stressful situations or before sleep).
Restrict dummy use to the months after breast-feeding has stopped and before speech develops (and certainly before adult teeth start to come through).
Make sure it is made from a single molded piece with holes stamped out to reduce choking risks.
Check the dummy nipple regularly - if it becomes brittle it could break up and be a choking risk.
Never tie a dummy around a child's neck because of the risk of strangulation.
Never fill dummies with fluid, especially sugary solutions.