"Can you get me a syringe with a 22 gauge needle? I want to give this lady a B12 shot."
Choosing a needle size for injection is one of the earliest "hands on skills" that doctors and nurses learn in order to give medication or draw blood with the least amount of discomfort for the patient. Needle gauge is measured in an inverse relationship so the larger number, the smaller the size of the needle. A 25 gauge needle is smaller than a 22 gauge needle which is smaller than a 20 gauge needle.
A 25 gauge needle is a fairly small needle that also comes in short lengths making them ideal for injections that are done frequently i.e. once or more times a day. They are very sharp and thin making the injection less painful. Insulin syringes have 25 gauge needles which is helpful since diabetes sometimes need to give themselves more than one injection a day.
Tuberculin syringes (used for TB skin tests) also have 25 gauge short needles and are sharp enough to only pierce the most superficial skin layer and allow injection of TB test fluid that creates that small little fluid blister. TB syringes with 25 gauge needles are the needle of choice for daily heparin shots so that the injection site bleeds very little after the shot and the short needle length puts the medication at the proper depth.
A 22 gauge needle is used for injections into muscle (intra-muscular) provided that the medication is fairly liquid like Vitamin B12 mentioned before, or into the layer above the muscle called the subcutaneous space. A 22 gauge needle is also longer 1- 1 inches in length, so it can reach the deeper space. All pain medication, many antibiotics and steroids are given through a 22 gauge needle.
Choose a 20 gauge needle if the medication is thicker or more vicious so the medication can flow more easily. The larger the gauge needle, the larger the opening inside the needle is for the fluid to flow through. This larger opening is sometime referred to as a larger "bore" needle. Needles that are 20 gauge are never used for subcutaneous injections, they are simply too big. If the medication being given is irritating a "Z" track technique is used to prevent leakage of the drug into the surrounding tissue.
Blood drawing requires more judgment and skill than giving an injection and paying attention to gauge is more important. The diameter of a blood cell is pretty small and in adults, blood is supposed to be drawn through a minimum of a 20 gauge needle since they have a larger opening or bore to prevent damage to the blood cells.
In smaller adults and young children, blood is frequently drawn with a 21, 22 or a 23 gauge butterfly needles (named from the plastic wings next to the needle). However, the risk of damaging the blood cells is higher and blood flows slower through the smaller gauge needles risking early clotting in the tube. This can make the lab results inaccurate so larger bore needles such as a 20 or 18 gauge are preferred in adults.
Ultimately, thorough knowledge of proper choice of needle gauge will ensure the patient experiences the least pain, the most efficient delivery of medication and the greatest benefit during injections or blood draws. No one ever likes to have to re-stick a patient because blood or medication won't flow properly so choose wisely when you make your first selection.