A heroin addict is addicted to the heroin both mentally and physically. The user not only craves the drug to feel good, but needs it to avoid withdrawal. This becomes a double edged sword for those individuals who try to end their dependence to the drug. During the three decades that heroin use was legal in the United States, heroin abuse occurred throughout the country and affected people of both sexes and of all social classes and races. When heroin was made illegal in 1924, however, abuse of the drug became most prevalent in the inner cities.

As a result, heroin abuse faded from the view of mainstream America, and addiction stopped being considered a problem that could affect nearly anyone. Instead, the drug would come to be dismissed by most Americans as something that could affect only the inner-city poor. However, during the 1960s and again in the 1990s, heroin abuse rose among the wealthy and the middle class. Heroin's renewed widespread popularity served as a reminder to American society that heroin abuse is a problem that does not discriminate along lines of socioeconomic standing, race, or age.

In the beginning, the heroin user will typically experiment just once or twice a week and in some cases less often, perhaps once a month or less. When a user is in the beginning stages of direct injection into a vein [mainlining], the most accessible veins are generally used. These are the veins located in the inner portion of the arm, near the elbow joint. This injection site is commonly called "the ditch" by users. If the person is mainlining, you may see scabs on the vein around this portion of the arm.

When someone becomes a heroin addict, they lose interest in their daily activities and find that their time is filled with using heroin or focused on obtaining more heroin. As their use progresses, addicts find that their tolerance continues to increase. This causes them to ingest more and more heroin to achieve the rush or high that they are looking for. As with other drugs of addiction, heroin addicts have trouble keeping their jobs and maintaining personal relationships. As their use becomes a priority in their lives their bank accounts begin to dwindle. It is not unusual for a heroin addict to spend upwards of $100-$200 dollars a day to feed their addiction.

At the early stages, there may be few signs of heroin addiction. At this point you would probably notice the injection sites. As the user progresses from experimentation to becoming a heroin addict, they will eventually reach a point where they shoot up at least once a day. At this point, several injection sites become noticeable. For example, over a 6 month period, the user will have shot up at least 180 times. This is when the needle marks manifest themselves as "tracks." Tracks are trails of tiny scabs that may extend 1-3 inches or more down the arm or leg in a straight line right over top of the vein.

Signs of a heroin addict include but are not limited to:

Pupils of the eyes are very small in reduced lighting conditions when pupils normally dilate
Track marks - If a person is right handed, he normally uses his right hand to inject himself in the left arm and left handed person normally injects into the right arm. As the tracks become progressively more visible, addicts will often wear long sleeves to hide the marks.
Collapsed veins - Injecting repeatedly into the same vein over and over again will cause the vein to collapse or "blow out". After a period of time, all veins in the arms may be blown out. At that point, the addict usually turns to veins located behind the knee or on the back of the hands.
Increase or decrease in appetite, changes in eating habits, unexplained weight loss or gain
Change in overall attitude / personality with no other identifiable cause
Changes in friends, new hang-outs, avoidance of old crowd, new friends are drug users
Change in activities, loss of interest in things that were important before
Drop in school or work performance, skips or is late to school or work
Changes in habits at home, loss of interest in family and family activities
Difficulty in paying attention, forgetfulness
Lack of motivation, energy, self-esteem, discipline
Bored, "I don't care" attitude
Defensiveness, temper tantrums, resentful behavior (everything's a hassle)
Unexplained moodiness, irritability, or nervousness
Violent temper or bizarre behavior
Paranoia – heroin addiction often causes users to become suspiciousness just about everyone and everything
Excessive need for privacy, keeps door locked or closed, won't let people in
Secretive or suspicious behavior
Car accidents, fender benders, household accidents
Chronic dishonesty
Trouble with the police
Unexplained need for money, can't explain where money goes, stealing
Change in personal grooming habits