Hypospray
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. ()

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. (; )

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. (; ; )
 
A hypospray could also be used to collect air samples in order to test it, or to condense air and store it for later use. (; )

See also 
Isomiotic hypospray
Spray applicator

Background information 
In the script for , hyposprays were commonly referred to as "hypo-guns." Similarly, in the final draft script of , a hypospray in that episode was consistently referred to as a "needleless injector". In , it was referred to as a "power-hypo".
The design of the  style of hypospray was inspired by a then-contemporary commercial inhaler. ()
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. 
Some say that the hypospray inspired the real-life "jet injector", but according to , 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  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  audio commentary, that 's constant administering of the Melvaran mud flea vaccine is the only time the hypospray seems to hurt.

External links
 
 

cs:Hyposprej
de:Hypospray
it:Ipospray
Medical equipment