The hypodermic syringe was invented in the mid-1800s. By the late 1800s and early 1900s in the United States, the public could purchase hypodermic needles and syringes legally, and opiates and cocaine were widely available. The Harrison Act in 1914 allowed narcotics to be dispensed only by a prescription from physicians or dentists. Several states also developed legislation preventing the sale of hypodermic syringes and needles without a prescription.
Most individuals who use injection drugs inject their drugs intravenously, but subcutaneous injection (ie, "skin-popping") is also common, and intramuscular injection may occur intentionally or when the individual misses the vein or the subcutaneous space. Injecting drug use is associated with many local and systemic complications for the individual and is also associated with the transmission of infectious diseases via needle sharing and sexual activity. The most commonly injected drug is heroin, but amphetamines, buprenorphine, benzodiazepines, barbiturates, cocaine, and methamphetamine also are injected. Any water-soluble drug may be injected. Treatment of individuals who use injection drugs may be complicated by social and political barriers to treatment and by a lack of resources for public health approaches to treatment.
Both illegal drug production and injecting drug use have been globalized in recent years. Injecting drug use has diffused to countries that formerly had little problem with it. Both injecting drug use and HIV infection can spread rapidly within a community. Introduction of and rapid increase in injecting drug use is believed to be responsible for dramatic increases in HIV infection in some areas. In China, in Central Asia, and in several countries of Eastern Europe, injecting drug use is the primary risk factor for HIV infection.
See related Medscape CME activity, The Dark Side of Drug Addicition.
When injecting a drug intravenously, the individual introduces a bolus of the drug into the vein, producing a rapid and powerful drug high. The onset of drug effects is about 15-30 seconds for the intravenous route and 3-5 minutes for the intramuscular or subcutaneous route. Drug effects from inhaling (ie, smoking) a drug begin in 7-10 seconds, and drug effects from intranasal use (ie, transmucosal absorption) begin in 3-5 minutes.
Injecting drug use causes medical problems by introducing pathogens and other contaminants into the body via shared needles and a lack of sterile preparation and injection techniques. Medical problems also arise from damage caused by the drugs themselves (eg, morbidity and mortality associated with drug overdose). The injected drugs also may not be pure; they may be cut with irritants, such as talc, lactate, or quinine.
Death from the direct toxic effects of a heroin overdose itself is usually associated with respiratory depression, coma, and pulmonary edema. Death from the direct effects of cocaine is often associated with cardiac dysrhythmias and conduction disturbances, leading to myocardial infarction and stroke.
According to the 2015 World Drug Report of the United Nations Office on Drugs and Crime (UNODC), a total of 246 million people used an illicit drug in 2013. 
The joint UNODC/WHO/UNAIDS/World Bank estimate for the number of people who inject drugs (PWID) worldwide for 2013 is 12.19 million (range: 8.48-21.46 million). This corresponds to 0.26% (range: 0.18-0.46 per cent) of the adult population aged 15-64 years. This estimate is based on reporting of information on current injecting drug use from 93 countries covering 84% of the global population aged 15-64. 
By far the highest prevalence of PWID continues to be found in Eastern and South-Eastern Europe, where 1.27% of the general population aged 15-64 is estimated to be injecting drugs, a rate nearly five times the global average. 
Morbidity and mortality may result from infection secondary to injecting drug use, sequelae of injection with adulterants added to the drug mixture, sequelae of the drug use itself, drug overdose, or violence associated with drug use.
About 1.65 million (range: 0.92-4.42 million) PWID were estimated to be living with HIV worldwide in 2013, which would correspond to 13.5% of PWID being HIV positive.  Besides direct transmission of HIV, injecting drug use also contributes to the spread of HIV infection by perinatal transmission and by sexual contact with individuals who do not inject drugs.  Injecting drug use is also associated with increased levels of high-risk sexual behavior.
Worldwide, 40-60% of individuals who use injection drugs are estimated to be positive for hepatitis B, and 60-70% are positive for hepatitis C virus (HCV). HCV rates are high even in countries with low HIV seroprevalence. Injecting drug use is responsible for approximately 60% of HCV infections in the United States. New HCV infections in the United States have declined since 1989  , but the incidence and prevalence of HCV remains high. The spread of HCV is rapid among those who are new to injecting drugs; in the United States, following initiation of injecting drug use, 50-80% become infected with HCV within 6-12 months. Reductions in risky injection-related practices among young users may improve both the burden of chronic HCV infection-related liver disease and elevated viral load-related poor treatment response.  Another option for reducing HCV transmission is encouraging users to use intranasal drugs as analternative to injection drugs.  For related information, see Medscape's Hepatitis B and Hepatitis C Resource Centers.
The mortality from all causes in individuals who use injection drugs is estimated to be 3-4% per year.
The National Survey on Drug Use and Health (NSDUH) found no differences in injection drug use reports by race or ethnicity in the United States in 2002 and 2003.
Combined 2002-2005 data from NSDUH indicate that US males were twice as likely as females to report injection drug use in the past year.
The Centers for Disease Control and Prevention in the United States reports that 24% of males and 25% of females living with HIV/AIDS in 2003 used injection drugs.  Another significant source of HIV infection for women is sex with partners who use injection drugs. An estimated 61% of AIDS cases in women can be attributed to injecting drug use or to sex with partners who use injection drugs. Females may use more shared injecting drug use equipment than males. 
Worldwide, 70-90% of those who use injection drugs are believed to be male.
The purity of heroin has been increasing, and its cost has been decreasing. Because of these factors and because of their initial desire to avoid injecting drug use, many adolescents and young adults in the United States and Europe using heroin for the first time try snorting, sniffing, or smoking heroin. New noninjecting heroin users risk making a transition to injecting drug use when their need for heroin use intensifies.
Among adolescents who inject drugs, early school truancy and expulsion may be a predictor of injecting drug use. A younger age of initiation into injecting drug use is associated with more frequent reports of risky drug use and sexual practices, as well as higher rates of HIV infection. 
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