An IV is a thin tube that is placed into a small vein to give you fluids or drugs. Sometimes, the vein becomes weak or damaged. Other times the tube comes out of the vein but is still in the tissue near the vein. An IV infiltration is when the fluids and drugs that are supposed to be going into the bloodstream build up in the tissue around the vein instead. The fluid leaks out and builds up in the tissue around the vein and causes pain. Also, the fluid or drugs do not get into the bloodstream to work as they were supposed to. Some fluids and drugs will harm the tissue when they leak. This is called an extravasation.