Needle size is determined by guage; the larger the guage number, the smaller the needle Needle size for an injection depends on the sight and the medication. For example: Insulin is given with a standard needle size of 25 gauge, and is given subcutaneously.
For a thin viscosity (resistance to flow) medication such as decadron, a 23 guage needle is adequate. 1/2 cc or less injections in the deltoid is preferred unless unusual circumstances warrant it, or the patient requests this sight after explanation. The muscle is not very big, and is a more painful sight for higher quantities of medication. An example of a deltoid injection would be tetanus toxoid.
Some triamcinolones or corticosteroids are of a thicker viscosity and most possibly should be given with a 21 guage needle. In the day of procain pennicillin, you had no choice but to use this size. Always use a luer-lock syringe if possible. This keeps the needle from jumping off the hypodermic syringe. The needle attaches to the body of the syringe via a screw type twist.
The easiest method for a quick IV infusion is with a "butterfly" needle. They are a standard 23 guage. For long-term IV infusion, an 18 guage angiocath is better, because you do not know when you may have to administer blood through this sight. In blood infusions, a smaller angiocath could cause hemolysis, or destruction of red cells; especially if you are infusing packed cells. The IV sight should be checked frequently for swelling, unusual pain and/or redness. This could indicate phlebitis or infiltration. Phlebitis is more common when irritating medications such as phenergan are administered through the sight. The IV should be rotated to a different location every three days.
Heparin locks are IV "tubes" filled with heparin, a blood thinner, for frequent IV injections directly into the area without the use of IV fluids to keep the vein open.
It should also be stressed that the nurse "pull back" on any IM or subcutaneous infusion, in order to determine if you have hit a vein. In this case, the needle should be withdrawn and the injection given in a different site with a new needle.
A good rule of thumb on injections in the gleuteus medius is to place your fingers on the anterior superior iliac spine; then, with your thumb extended, draw an imaginary half circle. If your injection is given within these bounds, your chances of hitting the sciatic nerve are greatly reduced.