Epigenetic modifications of histone tails in specific regions of the brain are of central importance in addictions, and much of the work on addiction has focused on histone acetylation.  Once particular epigenetic alterations occur, they appear to be long lasting "molecular scars" that may account for the persistence of addictions.
Cigarette smokers (about 21% of the US population) are usually addicted to nicotine.  After 7 days of nicotine treatment of mice, acetylation of both histone H3 and histone H4 was increased at the FosB promoter in the nucleus accumbens of the brain, causing 61% increase in FosB expression.  This would also increase expression of the splice variant Delta FosB.  In the nucleus accumbens of the brain, Delta FosB functions as a "sustained molecular switch" and "master control protein" in the development of an addiction.
About 7% of the US population is addicted to alcohol.  In rats exposed to alcohol for up to 5 days, there was an increase in histone 3 lysine 9 acetylation in the pronociceptin promoter in the brain amygdala complex.  This acetylation is an activating mark for pronociceptin.  The nociceptin/nociceptin opioid receptor system is involved in the reinforcing or conditioning effects of alcohol.
Cocaine addiction occurs in about 0.5% of the US population.  Repeated cocaine administration in mice induces hyperacetylation of histone 3 (H3) or histone 4 (H4) at 1,696 genes in one brain "reward" region [the nucleus accumbens (NAc)] and deacetylation at 206 genes.  At least 45 genes, shown in previous studies to be upregulated in the NAc of mice after chronic cocaine exposure, were found to be associated with hyperacetylation of H3 or H4.  Many of these individual genes are directly related to aspects of addiction associated with cocaine exposure.
In rodent models, many agents causing addiction, including tobacco smoke products, alcohol, cocaine, heroin  and methamphetamine, cause DNA damage in the brain.  During repair of DNA damages some individual repair events may alter the acetylations of histones at the sites of damage, or cause other epigenetic alterations, and thus leave an epigenetic scar on chromatin.  Such epigenetic scars likely contribute to the persistent epigenetic changes found in addictions.
In 2013, 22.7 million persons aged 12 or older needed treatment for an illicit drug or alcohol use problem (8.6 percent of persons aged 12 or older).