The British Small Animal Veterinary Association (BSAVA) maintains a database of adverse reactions to microchips. Since the database was started in 1996, over 4 million animals have been microchipped and only 391 adverse reactions have been reported. Of these reactions, migration of the microchip from its original implantation site is the most common problem reported. Other problems, such as failure of the microchip, hair loss, infection, swelling, and tumor formation, were reported in much lower numbers. For a chart summarizing the BSAVA reports, read the AVMA's literature review on Microchipping of Animals.