cigarette products are likely to become regular users and dependent on them. No data are available on the initiation of nicotine replacement product use among tobacco-naïve individuals or rates of diagnosable dependence, although these rates are likely to be low (Shiffman et al., 1998). The number of new NRT users among those attempting to quit was approximately 10% per year prior to over-the-counter (OTC) nicotine replacement products and 26% per year after OTC availability (Shiffman et al., 1998). Therefore, increased availability has led to increased use of these products among smokers, however, the rate of use still remains quite low. Furthermore, among smokers who use nicotine replacement products, persistent use tends to be low. Future research endeavors should concentrate on developing uniform methods and measures for assessing nicotine dependence so comparisons can be made across products and studies. The present measures are limited to assessing the extent of dependence and limited by being designed to diagnose other drugs of abuse and not specifically to diagnose nicotine dependence. In addition, as new products evolve, rates of initiation, regular use or persistent use and dependence, or progression to dependence as a result of experimentation should be assessed.
Models of Addiction. Several methods have been developed using clinical and animal models to determine the addiction potential or abuse liability of a drug. These include models of self-administration, drug discrimination, and conditioned drug placement. Models to examine withdrawal have also been developed. For humans, subjective responses to drugs can also be determined, although these responses may not necessarily be associated with actual drug-taking behavior.
When a drug is reinforcing, it is more likely to be self-administered or preferred compared to a control drug that has no abuse potential. The subject is exposed to a drug, typically, at varying doses and then required to choose between this particular drug and a control drug or an alternative reinforcer (e.g., sucrose for animals, money for humans), or between different doses of the drug.
In self-administration models, the animal is required to perform a particular maneuver, such as lever pressing, to obtain the drug, which is typically administered intravenously. This lever pressing could be based on a fixed ratio (a specific number of responses are required prior to drug delivery), a progressive ratio (more responses are required after each drug delivery), or an interval schedule (a certain time interval is necessary before drug delivery), or a combination of these. Scheduled reinforcement in response to environmental stimuli associated with drug administration are called second-order schedules (Goldberg et al., 1981). Drugs can be made