Hypodermic syringes are used with hypodermic needles to inject liquid or gases into body tissues, or to remove from the body. Injecting of air into a blood vessel is undesirable, as it may cause an air embolism; preventing embolisms by removing air from the syringe is the reason for the familiar image of holding a hypodermic syringe upside down, tapping it, and expelling a small amount of liquid before an injection into the bloodstream.
The barrel of a syringe is made of plastic or glass, and usually has graduated marks indicating the volume of fluid in the syringe, and is nearly always transparent. Glass syringes may be sterilized in an autoclave. However, most modern medical syringes are plastic with a rubber piston, because this type seals much better between the piston and the barrel and because they are cheap enough to dispose of after being used only once, reducing the risk of spreading blood-borne diseases. Re-use of needles and syringes has caused spread of diseases, especially HIV and Hepatitis among intravenous drug users. Syringes are commonly re-used by diabetics and this is safe, if the syringe is only used by one person.
Disposable medical syringes are sometimes used without a needle for orally administering liquid medicines to young children or animals because the dose can be measured accurately, and it is easier to squirt the medicine into the subject's mouth instead of coaxing the subject to drink out of a measuring spoon.