A hypospray (colloquially, hypo) was a medical device used to inject liquids into the body. The system used a noninvasive transport mechanism of compressed air to transfer the injectant from the device into the subdermal layer below the skin of the body, or artery. This was done without the use of a needle, ensuring that the skin was not punctured during use, thus reducing the risk of infection or pain at the site of injection. Various drugs could be used, inserted into the hypo in vials attached to the end of the instrument.
Doctor Phlox used hyposprays many times such as when he treated T'Pol's tension headache with an analgesic or Lieutenant Malcolm Reed's gastrointestinal distress symptoms. (ENT: "Breaking the Ice", "The Catwalk")
The 23rd century Federation hypospray resembled the intravenous needles of previous centuries, while the 24th century version of the device was more compact, employing an angled head and rounded tip to transfer the drug more easily. Controls at the injection head set the dosage to be injected. (TOS: "Amok Time"; TNG: "Haven")
The typical injection site used is the side of the neck, to the carotid artery, but the hypospray can inject even through clothing. Unlike hypodermic needles, the hypospray can be used on multiple patients without worries of spreading blood-borne illnesses. (TNG: "Angel One"; Star Trek: First Contact; VOY: "Parturition", "Endgame")
- In the script for TOS: "Where No Man Has Gone Before", hyposprays were commonly referred to as "hypo-guns." Similarly, in the final draft script of TOS: "Miri", a hypospray in that episode was consistently referred to as a "needleless injector". In TNG: "Where Silence Has Lease", it was referred to as a "power-hypo".
- The design of the TNG: style of hypospray was inspired by a then-contemporary commercial inhaler. (Star Trek: The Next Generation Companion 3rd ed., p. 11)
- Gates McFadden, who played Doctor Beverly Crusher, came up with the idea of injecting the hypospray at the base of the patient's neck because she felt it was the place where she would least want a shot. (citation needed • edit)
- Some say that the hypospray inspired the real-life "jet injector", but according to jet injector, the first real jet injector was invented in 1962; the first of the Star Trek stories were released in September 1966. In the real world, accidental jet injections by various industrial devices have been known as workshop accidents since the 19th century.
- A design patent was issued by the US Patent and Trademark Office to Rick Sternbach in 1991 for the "ornamental design" of the prop.
- A thoracic hypospray from Star Trek: Enterprise was sold off on the It's A Wrap! sale and auction on eBay. The hypospray resembled a needle, with the needle being spring-loaded for retraction to create an injection effect. 
- In the same auction, another model of hypospray was auctioned off. This medical device is identified as a Mk. VII hypospray and has a different design than the thoracic hypospray.
- J.J. Abrams acknowledged, on the Star Trek audio commentary, that Leonard McCoy's constant administering of the Melvaran mud flea vaccine is the only time the hypospray seems to hurt.