Adherence to NRTs and to other treatments for tobacco dependence is very low in the long term (< 40%), but it shows a strong positive correlation with better cessation outcomes. Unfortunately, these long-term cessation outcomes are still unsatisfactorily low (< 20%).The data presented in this chapter are based mainly on clinical trials and three population-based studies. Therefore the data on adherence and cessation rates presented here might be over-optimistic.

In order to improve the accuracy and comparability of measured adherence rates, further research is needed to establish explicit definitions of "adherence to treatment" and treatment dropout. A clearer understanding and distinction between the different factors that influence dropout is also needed.

The patterns of both adherence to therapy and cessation rates over time suggest that interventions for improving adherence would be more cost-effective the earlier they are introduced into the programme (i.e. during the first 3 weeks).

Surprisingly, lack of access to cheap NRTs has been reported as an important reason for smokers in developed countries failing to quit. This is unexpected because the cost of NRTs is usually equivalent to the cost of smoking. Substituting the demand at the same price should not be a reason not to adhere.

There are few data available for identifying effective adherence-promoting interventions, but the use of antidepressant drugs and psychosocial behavioural supports has shown good results. Studies to evaluate the cost-effectiveness of interventions for improving adherence are required.