In the DSM-5 (released in 2013), substance abuse and substance dependence have been merged into the category of substance use disorders and they no longer exist as individual diagnoses. Withdrawal
Withdrawal is the bodys reaction to abstaining from a substance upon which a person has developed a dependence syndrome. When dependence has developed, cessation of substance-use produces an unpleasant state, which promotes continued drug use through negative reinforcement; i.e., the drug is used to escape or avoid re-entering the associated withdrawal state. The withdrawal state may include physical-somatic symptoms (physical dependence), emotional-motivational symptoms (psychological dependence), or both. Chemical and hormonal imbalances may arise if the substance is not re-introduced. Psychological stress may also result if the substance is not re-introduced.Infants also experience substance withdrawal, known as neonatal abstinence syndrome (NAS), which can have severe and life-threatening effects. Addiction to drugs such as alcohol in expectant mothers not only causes NAS, but also an array of other issues which can continually affect the infant throughout their lifetime. Risk factors
Dependence potential
The dependence potential of a drug varies from substance to substance, and from individual to individual. Dose, frequency, pharmacokinetics of a particular substance, route of administration, and time are critical factors for developing a drug dependence. An article in The Lancet compared the harm and dependence liability of 20 drugs, using a scale from zero to three for physical dependence, psychological dependence, and pleasure to create a mean score for dependence. Selected results can be seen in the chart below.
Heroin-induced leukoencephalopathy often involves damage to cerebellar white matter, posterior cerebral white matter, posterior limb of internal capsule, and cerebellar peduncles. The occipital lobe is typically most affected though the frontal, parietal, and temporal lobes have shown involvement as well. Other toxins have been shown to extend damage to other structures of the brain, including the hippocampus, dorsal medulla, and brainstem. Treatment
With such a wide array of causes and unclear understanding of the pathophysiology, there is no known cure or treatment for the disease. In some cases of leukoencephalopathy induced by medications, such as methotrexate and metronidazole, the disease will reduce gradually once the medication is no longer distributed to the patient. Depending on the source of toxicity or pharmacological substance and severity of the white matter damage, many patients can have complete clinical recovery.Coenzyme Q and vitamin supplements, typically vitamin C and vitamin E, and other antioxidant therapies have been suggested to treat heroin-induced leukoencephalopathy patients. However, such treatments have rarely been trialed. See also
Leukoencephalopathy

References
Further reading
Hill MD, Cooper PW, Perry JR (January 2000). "Chasing the dragon – neurological toxicity associated with inhalation of heroin vapour: case report". CMAJ. 162 (2): 236–8. PMC 1232277. PMID 10674060.