Needle sharing is the practice of intravenous drug-users by which a syringe is shared by multiple individuals to administer intravenous drugs, and is a primary vector for diseases which can be transmitted through blood (blood-borne pathogens). People who inject drugs (PWID) are at an increased risk for Hepatitis C and HIV because of needle sharing practices. From 1933 to 1943, malaria was spread between users in the New York City area by this method. Afterward, the use of quinine as a cutting agent in drug mixes became common.
Needle-exchange programmes (NEP), a form of harm reduction policy, provide new needles to persons addicted to drugs in exchange for used ones in order to help control the spread of disease. In the United States, there are three distinct prohibitions on needle exchange programs at the federal level—the Ryan White CARE Act, the Substance Abuse and Mental Health Services Administration (SAMHSA) authorization, and the 1997 Labor-Health and Human Services (HHS) Education appropriations legislation. However, many states still provide the service despite the federal legislation, especially in large cities where intravenous drug use is a major health concern. A study in New York State found that during the course of 12 months NEP prevented a roughly calculated 87 infections of HIV by preventing needle sharing. Also, NEP have decreased the spread of HIV by one third to two fifths.
Discarding needles in regular household trash bins endangers children, pets, janitors, and waste management services personnel. Therefore, needles should be properly disposed of in a FDA-cleared sharps container that is both puncture- and leak-resistant. It is important to properly dispose of used needles because it can be difficult to tell if a needle has ever been used before. By not disposing of needles in a safe way, users may expose others or themselves to a needle stick injury. If a sharps container is not available, needles can often be brought to local law enforcement, hospitals, or drug stores for safe collection and disposal. In addition, needles can be mailed in specialized containers to a mail-back program for safe disposal. In the health care setting, use of blunt-end needles can minimize the risk of needle stick injuries 
Someone who has been injected with a medicine or drug using a syringe or needle that has been used by someone with HIV may be at risk of getting HIV as well. However, because a person may not know whether someone has HIV or not, as a precaution, an individual should never reuse a needle or syringe. An individual may never know whether someone else or himself has a terminal or severe infection that is contagious. Therefore, it is never safe to share a needle or syringe with another individual. According to a study done by New Haven Connecticut’s needle exchange program, people returning needles that had not originated from the facility had a rate up to 67.5% of showing positive for HIV. Their assumption was that people bringing in “street needles” were shared among other people prior to bringing them to the program. In addition to HIV, Hepatitis B and Hepatitis C are also commonly transmitted through needle sharing.
A simple prick or accidentally touching a used infected needle can put someone at risk of acquiring Hepatitis B, Hepatitis C, or HIV. Hepatitis B is the easiest to contract, followed by Hepatitis C, followed by HIV from discarded needles. Almost 50% of people who participate in IVDU have Hepatitis C. Not only are blood borne diseases passed via needle sharing, but so are bacterial infections that can ultimately cause sepsis. Additionally, improper disposal of hospital needles can expose drug resistant organisms to the outside environment.
Much research has been done on risk factors that may predispose an individual to needle sharing in an effort to improve the effectiveness of NEPs and other harm reduction programs for PWID. Within the United States, needle sharing behavior is positively correlated to individuals who are of lower socioeconomic status, younger than 45 years old, male, and unemployed. Additionally, people of a minority race or ethnicity are often at increased risk of needle sharing, possibly due to lower levels of health education.