Titanium is what the implant is made from. This is due to its biocompatibility and ability for the body to accept the metal, not reject it. However, if the mouth is extremely acidic, the implant may corrode. If this happens, and you have metal fillings in your mouth, this could cause galvanic corrosion.
Infections may occur following surgery, but patients are usually prescribed antibiotics after treatment to help avoid this occurring. If proper after-care is followed, this should not be a problem.
There is also the risk of tissue necrosis of the cells in the flaps of gum around the implant. This happens in 5% of cases. There is also the possibility that the gums next to the dental implant may recede, exposing the abutment that holds the tooth in place. Improper drilling protocols or over torqueing the implant may cause bone necrosis which could lead to implant failure.