I agree with Dr Coyner. Its best to make sure that there is not a serious cause to the staining. As she has stated this is a common problem in small dogs. In fact it is a problem in many small dogs, not just white ones. Its just that its easy to see the stain on the white fur and its not as noticeable on darker dogs. The stain is actually a reaction between the sunlight and the tears. If after you have visited with your veterinarian and determined that its not a serious problem, you may want to try a product called Angel Eyes. We have had some success with it.
Hope this helps,
Dr Dean Vicksman
Tear staining is common in small white dogs, and is often secondary to plugged tearducts, or to small hairs on the eyelid margins wicking the tears onto the fur. I recommend that you have your dog evaluated by a veterinarian, and possibly a veterinary ophthalmologist to find and treat the cause of the tear staining.
Kimberly Coyner, DVM