For intravenous users of heroin (and any other substance), the use of non-sterile needles and syringes and other related equipment leads to the risk of contracting blood-borne pathogens such as HIV and hepatitis, as well as the risk of contracting bacterial or fungal endocarditis and possibly venous sclerosis.
Poisoning from contaminants added to "cut" or dilute heroin
Chronic constipation
Heroin-induced toxic leukoencephalopathy (very rare, smokers only, the causal reason is currently unknown)
Addiction and constantly growing tolerance. Like all opiates and opioids, long term use can lead to physical addiction.
Decreased kidney function. (although it is not currently known if this is due to adulterants used in the cut)
Many countries and local governments have begun funding programs that supply sterile needles to people who inject illegal drugs in an attempt to reduce these contingent risks and especially the contraction and spread of blood-borne diseases. The Drug Policy Alliance reports that up to 75% of new AIDS cases among women and children are directly or indirectly a consequence of drug use by injection. But despite the immediate public health benefit of needle exchanges, some see such programs as tacit acceptance of illicit drug use. The United States federal government does not operate needle exchanges, although some state and local governments do support needle exchange programs. Needle exchanges have been instrumental in arresting the spread of HIV/AIDS in many communities with a significant heroin using population,[citation needed] Australia being a leader due to its early inception of needle exchanges. Needle exchange programs have also been attributed to saving the public significant amounts of tax money by preventing medical costs which would have been required otherwise for the treatment of diseases spread through the practice of sharing and reusing needles.
A heroin overdose is usually treated with an opioid antagonist, such as naloxone (Narcan), which has a high affinity for opioid receptors but does not activate them. This blocks heroin and other opioid antagonists and causes an immediate return of consciousness and the beginning of withdrawal symptoms when administered intravenously. The half-life of this antagonist is usually much shorter than that of the opiate drugs it is used to block, so the antagonist usually has to be re-administered multiple times until the opiate has been metabolized by the body.
Depending on drug interactions and numerous other factors, death from overdose can take anywhere from several minutes to several hours due to anoxia because the breathing reflex is suppressed by µ-opioids. An overdose is immediately reversible with an opioid antagonist injection. Heroin overdoses can occur due to an unexpected increase in the dose or purity or due to diminished opiate tolerance. However, most fatalities reported as overdoses are probably caused by interactions with other depressant drugs like alcohol or benzodiazepines.
The LD50 for a physically addicted person is prohibitively high,[citation needed] to the point that there is no general medical consensus on where to place it. Several studies done in the 1920s gave users doses of 1,600–1,800 mg of heroin in one sitting, and no adverse effects were reported. This is approximately 16–18 times a normal recreational dose.[citation needed] Even for a non-user, the LD50 can be placed above 350 mg[citation needed] though some sources give a figure of between 75 and 375 mg for a 75 kg person.
Street heroin is of widely varying and unpredictable purity. This means that the user may prepare what they consider to be a moderate dose while actually taking far more than intended. Also, those who use the drug after a period of abstinence have tolerances below what they were during active addiction. If a dose comparable to their previous use is taken, an effect greater to what the user intended is caused, in extreme cases an overdose could result.
It has been speculated that an unknown portion of heroin related deaths are the result of an overdose or allergic reaction to quinine, which may sometimes be used as a cutting agent.
A final source of overdose in users comes from place conditioning. Heroin use, like other drug using behaviors, is highly ritualized. While the mechanism has yet to be clearly elucidated, it has been shown that longtime heroin users, immediately before injecting in a common area for heroin use, show an acute increase in metabolism and a surge in the concentration of opiate-metabolizing enzymes. This acute increase, a reaction to a location where the user has repeatedly injected heroin, imbues him or her with a strong (but temporary) tolerance to the toxic effects of the drug. When the user injects in a different location, this place-conditioned tolerance does not occur, giving the user a much lower-than-expected ability to metabolize the drug. The user's typical dose of the drug, in the face of decreased tolerance, becomes far too high and can be toxic, leading to overdose.
A small percentage of heroin smokers may develop symptoms of toxic leukoencephalopathy. This is believed to be caused by an uncommon adulterant that is only active when heated. Symptoms include slurred speech and difficulty walking.

- DrugTesters