In most, the answer is a combination of the three factors. These were slightly unusual circumstances of drug use. In the war zone, heroin use was associated with enduring hardship, uncertainty and constant risks of death and injury without the support of home and family. Addictive substances are usually defined by their ability to be associated with continued, compulsive use despite evident adverse consequences, as well as the occurrence of reproducible, objective signs of withdrawal when the use of the substance is curtailed. Other features of addiction are 'salience' and 'tolerance'.
Salience implies a pre-occupation with the drug and a narrowing of the drug use repertoire.
Tolerance may cause the quantities consumed to increase substantially without a corresponding increase in the effects. As the addiction becomes more entrenched, so the variety of drugs taken becomes more limited and the circumstances of drug use become more regular and predictable. The major difference health-wise is that the tobacco yields the greater measurable harm of these three addictive drug groups. Some drugs appear to have relatively minor or no direct adverse consequences in most users, but can cause serious problems in a small number. Stimulants such as coffee, tea, and coca leaf may fall into this category.