Final answer:
Stannous fluoride can cause surface stains on teeth and, when present in excess due to high natural levels in drinking water, can lead to skeletal fluorosis. This bone condition results in stiffened joints and thickened bones, affecting mobility and the thyroid gland.
Step-by-step explanation:
A negative side effect of stannous fluoride is the potential to cause dental surface stains. While Fluoride in various forms, including stannous fluoride is commonly used in toothpaste for cavity prevention and the control of gingivitis, when used in excess it could have severe health repercussions. One of the most serious side effects, particularly from high natural fluoride levels in drinking water, is a bone disease known as skeletal fluorosis. This condition can cause joints to stiffen and bones to thicken, leading to impaired mobility and potential thyroid gland issues. It is such a significant issue that the US Environmental Protection Agency has capped fluoride levels in drinking water to a maximum of 4 ppm (4 mg/L) to prevent this disease.