According to HealthGuidance, causes of black spots under the tongue include tongue piercings, hyper-pigmentation, excessive smoking and drinking, and oral cancer. Some of the causes are harmless, while others require medical attention.
HealthGuidance notes that tongue piercings can leave black spots under the tongue as a result of a loss of pigmentation in the pierced area. An individual can also have an abundance of pigmentation, known as hyperpigmentation, on the tongue, which can lead to dark spots. Hyperpigmentation can either develop on its own or as a result of a genetic trait. Black spots on the tongue caused by hyperpigmentation are harmless, but they can be removed with a laser.
Oral cancer is a rare cause of black spots on the tongue, according to HealthGuidance. One way that an individual can tell whether or not the spots are the result of cancer is if he is also experiencing difficulty swallowing and has lumps in the mouth, both of which are symptoms of oral cancer.
HealthGuidance also notes that excessive smoking and drinking can lead to a condition known as hairy tongue, in which an individual has a fungus on the tongue that looks like black spots. The condition goes away on its own if the individual stops drinking and smoking.