THE HIGH
The reason most heroin addicts first started using is because of the "high" it promises. Like most drugs, the high triggers an alluring rush of dopamine in the brain, and like most opiates, a heroin user experiences intense euphoria - a feeling of happiness, confidence, or well-being sometimes exaggerated in pathological states as mania (dictionary.com). A common effect among heroin users is the delusion that parasites are attached to the inside of their skin, moving around, and while under the influence of heroin, many users will attempt to scratch, or even carve the imaginary pests out of their skin.
THE LONG TERM
While heroin offers an attractive high, it also carries with it long term negative consequences. Such effects include dramatic weight loss, gum inflammation, respiratory illness, memory loss, if injected vein collapse; and many users also report having delusions of parasites crawling shallowly beneath the skin, causing them to scratch themselves uncontrollably, and develop sores on their faces and forearms. Additionally due to heroin's illegal status, illicit distributors of the drug may "cut" it with household products, in order to increase their profit, according to the BBC, substances such as talcum powder, and even brick investment have been found in street grade heroin; these cuts can increase the damage caused by the already dangerous drug.