MODEL RELEASED. Inserting cannula. A nurse inserts a Venflon cannula into a vein on the back of a patient's hand. A cannula is a thin tube with an open, unsharpened tip that is inserted into the body and used to introduce or withdraw fluids. Once the cannula is in place, it can be secured with tape and left for hours or even days. This cannula was used for intravenous delivery of drugs during an operation. In surgery a cannula is used to deliver a pre-operative anaesthetic that induces unconsciousness.