A black hairy tongue,as is the condition apparent in the picture is caused by bacterial or yeast growth in the mouth which may be attributed to a no of factors,ranging from lack of good oral hygeine,to regular use of Oral lozenges/mouthwashes,regular use of antibiotics,drinking too much of tea or coffee to name a few. Practicing a good oral hygeine and regular follow up with your dentist will definitely help. Regards, Dr.Sanchit Talwar MD (Skin & VD), Consultant Dermatologist, Dr Talwar's Skin, Laser and Cosmetic Clinic CHANDIGARH.