An intradermal injections consists in the introduction, via needle, of little amounts of fluid into the skin. A subcutaneous injection, on the other side, introduce the fluid under the skin. But why would you want to put a fluid in the skin layers and not under the skin?
Well, the main reason is that a subcutaneous injection has a completely different purpose from an intradermal injection. As a matter of fact the latter is mainly used to check if the patient is affected by some kind of specific allergies. Even though it can also be used for different things, such as controlling whether a person contracted tuberculosis.
Because the fluid must be expelled between the layers of the skin the syringe consists of a very thin needle of 27 or 25 gauge and 3/4 to 3/8 inch. The needle is not only narrow, but also very short. When a shot is give the syringe is usually nearly parallel to the forearm.
Even though intradermal injections are normally given in the inner palm-side surface of the forearm, there can be some exceptions depending on what kind of test has to be performed.
Because most of the intradermal injections are given with the purpose of analyzing whether a certain skin reaction occurs or not, there might be some tests where the patient will have to return to the physician after a few days in order to confirm whether or not the reaction took place. This happens in case of a test for tuberculosis. However some other kind of reactions can be quite fast, in that case there will be no need for the patient to return a few days later to check everything.
Because of this it is always recommended not to rub the shot site, so that everything can be clear when analyzed.
Some people find intradermal injections more annoying than subcutaneous injections, however these kind of injections are not particularly painful.