While most natural teeth can be whitened, there are a number of exceptions. Unfortunately, only real (natural) teeth can we whitened significantly. Caps, crowns, veneers, etc can sometimes benefit slightly from a teeth whitening treatment, but not much really, and could only be taken back to their original color. People whose stains are derived from the antibiotic tetracycline or from fluorosis will see little or no improvement, but in some cases a little improvement means a lot to them. The more yellow your teeth are, the easier they are to whiten and the greater the improvement you will obtain.