Brushing the tongue when brushing your teeth has been touted as a cure for bad breath caused by bacteria that form on your tongue. This cleaning of the tongue is supposed to both improve bad breath and lessen the risk of infection in periodontal openings while reducing bacterial population. While the process seems fairly benign, there are possible side effects from brushing improperly or too harshly. A May 2003 study by the Department of Periodontology, Academic Centre for Dentistry, Amsterdam, the Netherlands reported on by Wiley Interscience concluded there was reduced coating and increased sense of taste after brushing, but only minor lessening of bacteria on the tongue surface.
Tongue brushing can result in pressing bacteria further into the tongue surface (filliform) rather than removing it. Brushing without scraping had little positive effect and could actually prolong the bacterial propagation and resulting bad breath according to Dr. Dan Peterson on the Family Gentle Dental website.
Damage to the taste buds and opening the way for possible infection are side effects of brushing the tongue too vigorously or with harsh bristles.