Veneers are a thin coating of either porcelain or composite (plastic) glued to the enamel surface of the teeth to improve appearance (by changing the shape, size or colour).
Here are some important considerations that should be made before choosing to have a veneer/multiple veneers:
1) What material will be used: composite tends to stain more easily but is much easier to repair than porcelain. Porcelain will be more suitable for patients with a highly staining diet (lots of tea, coffee, red wine, cigarettes).
2) Some tooth preparation (trimming) may be required: very few cases of veneers can be done "no prep" because purely adding material to the tooth may make the tooth too bulky.
3) Veneers will need replacing: if you like to chew on very hard food (bones, ice, shellfish) then your veneers will chip or break much more frequently. The veneer may have to be completely removed from the tooth and redone.
As you age, the veneers done in your 20s for example, may no longer suit a more mature face in your 50s. This may be another reason for veneer replacement.
4) Consider having orthodontic retainers after the veneer/veneers are completed: the underlying teeth can migrate and cause misalignment of the veneers as time goes by.
5) Less than ideal results: the final veneers may not be what you have envisioned (in terms of colour, position, shape, size or alignment) if proper pre-planning has not been carried out.
6) You may need other treatment (braces, gum alteration or teeth whitening) prior to having veneers for the best result.