In the Star Trek universe, the hypospray was developed by the mid-22nd century, as it is featured in Star Trek: Enterprise. Many people, such as Dr. Crusher in Star Trek: The Next Generation and The Doctor in Star Trek: Voyager, are seen to use it.
The real-life jet injector is usually applied at the top of the arm, but the fictional hypospray is sometimes applied on the neck. Presumably when used in the neck it delivers the medication intravenously or intraarterially and when used on the arm it delivers intramuscularly. The hypospray can also be applied through clothing.
The hypospray is extremely versatile as the medicine vials can be quickly swapped out from the bottom of the hypospray. As the hypospray is bloodless, it is not contaminated by use. This allows it to be used on many people until the supply of medicine runs out.
The concept of the hypospray was developed when producers on the original Star Trek series discovered that NBC’s broadcast standards and practices prohibited the use of hypodermic syringes to inject medications; the needleless hypospray sidestepped this issue. The prop used in the original series appeared to be a modified fuel injector for a large automotive diesel engine.