Int. J. Environ. Res. Public Health 2012, 9 193 Keywords: tobacco; alcohol; opiates; cocaine; cannabis; addiction 1. Introduction A recent review considering the definition of addiction highlighted five essential elements, which mainly addressed issues of the individuals and their behaviour and interaction with addictive stimuli [1]. Addictive interactions also involve the addictive stimulus itself, and the interactions are often described as strong or severe, without clear explanation of what strong or severe means. Here we hypothesis that it may be possible to assess the strength of an addiction in terms of how hard it is to break, and the severity of an addiction in terms of adverse physical, psychological and social effects, the duration of the addiction, and the number of people affected. It should be emphasised that a dictionary definition of hypothesis is “a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation”; that is exactly the case here, where there is no significant prior evidence, merely an assumption that this might be reasonable. There is an implied assumption that addictions to different substances vary in strength from weak (easier to stop) to strong (harder to stop), though explicit definitions are lacking. Using the data of the National Epidemiologic Survey on Alcohol and Related Condition, Lopez-Quintero et al. have shown that the probability of a transition from first use to dependence was greater with nicotine followed by alcohol, cocaine, and cannabis [1], and that after the onset of dependence, remission appeared usually first for cocaine, followed by cannabis, alcohol, and finally nicotine [2]. Another way to explore the strength of addictions is to measure cessation rates found with placebo or no treatment controls; a weaker addiction would have a higher cessation rate than a stronger addiction. Comparing different forms of addiction in order to categorise the strength of addiction is not easy for reasons concerning participants, setting, and details of study design: 1. Motivation to quit and the anticipated benefit are predictors of substance cessation [3], and treatment intensity affects outcome, as shown in smoking cessation meta-analysis [4]. These can be linked and differ markedly between addictions. Alcohol dependent subjects entering treatment with current social and psychological distress that improves dramatically in days or weeks of inpatient treatment are quite different from smokers with no apparent immediate benefit trying to quit alone and at home. 2. Individual circumstances differ. An example might be individuals recruited in primary care for a short intervention for smoking cessation, compared to those in hospital with a recent diagnosis of smoking-related disease [5]. Recent smoking cessation trials have shown quit rates to be better after admission for coronary heart disease [6,7]. 3. Types and intensity of intervention may be an influence, divided mainly between psychosocial interventions (cognitive behavioural, support psychotherapy) and drugs of various sorts. 4. Cessation can be defined in a number of ways. Best is to have some objective test, like breath carbon monoxide or urine cotinine for nicotine, benzoylecgonine or opioid metabolites for illicit drugs, and biochemical tests for alcohol. Self-reported abstinence is likely to be less reliable, unless confirmed by a significant other.