Heroin is exceedingly addictive, quickly producing tolerance and dependence. Although heroin is even more effective as a painkiller than morphine and codeine, it is so highly addictive that its use is illegal. Methadone is a synthetic opiate that is used to break addiction to heroin (and replace it with addiction to methadone).

Short Term Heroin Effects

Soon after injection (or inhalation), heroin crosses the blood-brain barrier. In the brain, heroin is converted to morphine and binds rapidly to opioid receptors. Abusers typically report feeling a surge of pleasurable sensation, a "rush." The intensity of the rush is a function of how much drug is taken and how rapidly the drug enters the brain and binds to the natural opioid receptors. Heroin is particularly addictive because it enters the brain so rapidly. With heroin, the rush is usually accompanied by a warm flushing of the skin, dry mouth, and a heavy feeling in the extremities, which may be accompanied by nausea, vomiting, and severe itching.

After the initial effects, abusers usually will be drowsy for several hours. Mental function is clouded by heroin's effect on the central nervous system. Cardiac function slows. Breathing is also severely slowed, sometimes to the point of death. Heroin overdose is a particular risk on the street, where the amount and purity of the drug cannot be accurately known.

• Analgesia (reduced pain)
  
• Brief euphoria (the "rush" or feeling of well-being)
  
• Nausea
  
• Sedation, drowsiness
  
• Reduced anxiety
  
• Hypothermia
  
• Reduced respiration; breathing difficulties
  
• Reduced coughing
  
• Death due to overdose - often the exact purity and content of the drug is not known to the user. An overdose can cause respiration problems and coma

Long Term Heroin Effects

One of the most detrimental long-term effects of heroin is addiction itself. Addiction is a characterized by compulsive drug seeking and use, and by neurochemical and molecular changes in the brain. Heroin also produces profound degrees of tolerance and physical dependence, which are also powerful motivating factors for compulsive use and abuse. As with abusers of any addictive drug, heroin abusers gradually spend more and more time and energy obtaining and using the drug. Once they are addicted, the heroin abusers' primary purpose in life becomes seeking and using drugs. The drugs literally change their brains.

• Tolerance: more and more drug is needed to produce the euphoria and other effects on behavior.
• Addiction: psychological and physiological need for heroin. People are driven to get more heroin and feel bad if they do not get it. People begin to crave heroin 4 to 6 hours after their last injection.
• Withdrawal: About 8-12 hours after their last heroin dose, addicts' eyes tear, they yawn and feel anxious and irritable. Excessive sweating, fever, stomach and muscle cramps, diarrhea and chills can follow several hours later. These withdrawal symptoms can continue for 1 to 3 days after the last dose and can last 7 to 10 days. In some cases, full recovery can take even longer.

Other Heroin Effects

Medical consequences of chronic heroin abuse include scarred and/or collapsed veins, bacterial infections of the blood vessels and heart valves, abscesses (boils) and other soft-tissue infections, and liver or kidney disease. Lung complications (including various types of pneumonia and tuberculosis) may result from the poor health condition of the abuser as well as from heroin's depressing effects on respiration. Many of the additives in street heroin may include substances that do not readily dissolve and result in clogging the blood vessels that lead to the lungs, liver, kidneys, or brain. This can cause infection or even death of small patches of cells in vital organs. Immune reactions to these or other contaminants can cause arthritis or other rheumatologic problems.

Of course, sharing of injection equipment or fluids can lead to some of the most severe consequences of heroin abuse-infections with hepatitis B and C, HIV, and a host of other blood-borne viruses, which drug abusers can then pass on to their sexual partners and children.

• HIV/AIDS - due to sharing of needles
• Poisoning - from the addition of toxin to the drug
• Hepatitis - liver damage
• Skin infections - from repeated intravenous injections
• Other bacterial and viral infections
• Increase risk of stroke
• Collapsed veins
• Lung infections

Not all of the mechanisms by which heroin and other opiates affect the brain are known. Likewise, the exact brain mechanisms that cause tolerance and addiction are not completely understood. Opiates stimulate a "pleasure system" in the brain. This system involves neurons in the midbrain that use the neurotransmitter called "dopamine." These midbrain dopamine neurons project to another structure called the nucleus accumbens which then projects to the cerebral cortex. This system is responsible for the pleasurable effects of heroin and for the addictive power of the drug. Other neurotransmitter systems, such as those related to endorphins, are also likely to be involved with withdrawal from and tolerance to heroin.