Cannula
A short plastic tube placed into a vein to give fluids or medicines.
What it means
A cannula is a short plastic tube inserted into a vein to give fluids, medicines or blood. It is the line taped to the back of the hand or forearm in hospital, and is what people usually mean by "a drip".
A needle is used to place it and then withdrawn, leaving only soft plastic in the vein — which is why a cannula does not hurt once sited.
Nasal cannulae are a separate thing entirely: soft prongs sitting at the nostrils to deliver oxygen.
In practice
Colour-coded by size, with larger bores allowing faster fluid delivery.
Phlebitis — inflammation of the vein around a cannula — is the commonest complication, and is why cannulae are re-sited every 72 hours or so.
Why it matters
Redness, pain or swelling spreading up the arm from a cannula site, or after one has been removed, should be assessed rather than watched.
Being cannulated does not imply serious illness — it is routine before many procedures, often placed purely as a precaution and never used.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026