According to WebMD, reasons why teeth get stained brown or exhibit other discoloration include the use of tobacco or consuming certain foods and drinks and even some fruits and vegetables. Poor oral hygiene and diseases are other causes of brown spots on teeth.
Medline Plus states that there are three main reasons that cause brown spots on teeth: infections, environmental factors and congenital illnesses. The latter includes diseases that adversely affect the enamel and the levels of calcium and protein in the tooth enamel. Metabolic diseases, medicines taken by the mother while pregnant or a bad case of jaundice when a child is born can also cause brown spots on the teeth.
WebMD lists certain items of food and drink such as apples, potato, tea, coffee and wine as reasons for brown discoloration on the teeth. Failing to clean teeth properly and remove plaque can also results in stained teeth over time, as can smoking or chewing tobacco. Certain antibiotics given to children when their permanent teeth are forming and some mouthwashes and medicines for high blood pressure can cause brown spots. Other factors that cause this condition include radiation and chemotherapy, old age and exposure to high levels of fluoride, as stated by WebMD.