Markov chain modeling of initiation and demand: the case of the U.S. cocaine epidemic

Cocaine's fall and marijuana's rise: questions and insights based on new estimates of consumption and expenditures in US drug markets

AIMS

Drug policy strategies and discussions often use prevalence of drug use as a primary performance indicator. However, three other indicators are at least as relevant: the number of heavy users, total expenditures and total amount consumed. This paper stems from our efforts to develop annual estimates of these three measures for cocaine (including crack), heroin, marijuana and methamphetamine in the United States.

METHODS

The estimates exploit complementary strengths of a general population survey (National Survey on Drug Use and Health) and both survey and urinalysis test result data for arrestees (Arrestee Drug Abuse Monitoring Program), supplemented by many other data sources.

RESULTS

Throughout the 2000s US drug users spent in the order of $100 billion annually on these drugs, although the spending distribution and use patterns changed dramatically. From 2006 to 2010, the amount of marijuana consumed in the United States probably increased by more than 30%, while the amount of cocaine consumed in the United States fell by approximately 50%. These figures are consistent with supply-side indicators, such as seizures and production estimates. For all the drugs, total consumption and expenditures are driven by the minority of users who consume on 21 or more days each month.

CONCLUSIONS

Even for established drugs, consumption can change rapidly. The halving of the cocaine market in five years and the parallel (but independent) large rise in daily/near-daily marijuana use are major events that were not anticipated by the expert community and raise important theoretical, research, and policy issues.

Does tougher enforcement make drugs more expensive?

AIMS

To review empirical research that seeks to relate marginal increases in enforcement against the supply of illicit drugs to changes in drug prices at the level of the drug supply system being targeted.

METHOD

Review of empirical studies.

FINDINGS

Although the fact of prohibition itself raises prices far above those likely to pertain in legal markets, there is little evidence that raising the risk of arrest, incarceration or seizure at different levels of the distribution system will raise prices at the targeted level, let alone retail prices. The number of studies available is small; they use a great variety of outcome and input measures and they all face substantial conceptual and empirical problems.

CONCLUSION

Given the high human and economic costs of stringent enforcement measures, particularly incarceration, the lack of evidence that tougher enforcement raises prices call into question the value, at the margin, of stringent supply-side enforcement policies in high-enforcement nations.

Mapping the spread of methamphetamine abuse in California from 1995 to 2008

OBJECTIVES

From 1983 to 2008, the incidence of methamphetamine abuse and dependence (MA) presenting at hospitals in California increased 13-fold. We assessed whether this growth could be characterized as a drug epidemic.

METHODS

We geocoded MA discharges to residential zip codes from 1995 through 2008. We related discharges to population and environmental characteristics using Bayesian Poisson conditional autoregressive models, correcting for small area effects and spatial misalignment and enabling an assessment of contagion between areas.

RESULTS

MA incidence increased exponentially in 3 phases interrupted by implementation of laws limiting access to methamphetamine precursors. MA growth from 1999 through 2008 was 17% per year. MA was greatest in areas with larger White or Hispanic low-income populations, small household sizes, and good connections to highway systems. Spatial misalignment was a source of bias in estimated effects. Spatial autocorrelation was substantial, accounting for approximately 80% of error variance in the model.

CONCLUSIONS

From 1995 through 2008, MA exhibited signs of growth and spatial spread characteristic of drug epidemics, spreading most rapidly through low-income White and Hispanic populations living outside dense urban areas.

A systems approach to college drinking: development of a deterministic model for testing alcohol control policies

OBJECTIVE

The misuse and abuse of alcohol among college students remain persistent problems. Using a systems approach to understand the dynamics of student drinking behavior and thus forecasting the impact of campus policy to address the problem represents a novel approach. Toward this end, the successful development of a predictive mathematical model of college drinking would represent a significant advance for prevention efforts.

METHOD

A deterministic, compartmental model of college drinking was developed, incorporating three processes: (1) individual factors, (2) social interactions, and (3) social norms. The model quantifies these processes in terms of the movement of students between drinking compartments characterized by five styles of college drinking: abstainers, light drinkers, moderate drinkers, problem drinkers, and heavy episodic drinkers. Predictions from the model were first compared with actual campus-level data and then used to predict the effects of several simulated interventions to address heavy episodic drinking.

RESULTS

First, the model provides a reasonable fit of actual drinking styles of students attending Social Norms Marketing Research Project campuses varying by "wetness" and by drinking styles of matriculating students. Second, the model predicts that a combination of simulated interventions targeting heavy episodic drinkers at a moderately "dry" campus would extinguish heavy episodic drinkers, replacing them with light and moderate drinkers. Instituting the same combination of simulated interventions at a moderately "wet" campus would result in only a moderate reduction in heavy episodic drinkers (i.e., 50% to 35%).

CONCLUSIONS

A simple, five-state compartmental model adequately predicted the actual drinking patterns of students from a variety of campuses surveyed in the Social Norms Marketing Research Project study. The model predicted the impact on drinking patterns of several simulated interventions to address heavy episodic drinking on various types of campuses.

Effective drug policy: a new approach demonstrated in the Drug Policy Modelling Program

The aim of this paper is to describe a new comprehensive approach to studying illicit drug policy - one that integrates evidence, disciplinary approaches, drug use behaviours and policy making processes. The methods described here include systematic reviews of the evidence, studies of the ways in which policy decision-making actually occurs, and the use of modelling approaches that can explicate the multi-dimensional nature of drug policy responses and their dynamic interactions. The approach described has the potential to facilitate new drug policy that would not have been possible or apparent through the sole study of one aspect of drug policy, such as the evidence-base or the political context or the economics of drug markets. We believe this approach may be more likely to produce strategic drug policy because it reflects the richness and complexity of the real world of drug use, and drug policy. The purpose of employing an integrative methodology is to create the potential for new drug policy insights, ideas and interventions - not restricted to one body of evidence, nor to accidental or fortuitous policy-making processes.

Dilemmas in harm minimization

This paper discusses the dilemmas inherent in pursuing a philosophy of (drug) harm minimization. The dilemmas arise (i) because all drug control policies produce harms as well as benefits; (ii) because many of these harms and benefits cannot be measured; and (iii) because even when they can be measured, judgements about what harms matter the most are irreducibly political. The paper concludes by proposing that the interests of drug policy might be better served if we abandoned the idea of an overarching goal in favour of a set of goals dealing with specific identifiable problems.

Social epidemiology and complex system dynamic modelling as applied to health behaviour and drug use research

A social epidemiologic perspective considers factors at multiple levels of influence (e.g., social networks, neighbourhoods, states) that may individually or jointly affect health and health behaviour. This provides a useful lens through which to understand the production of health behaviours in general, and drug use in particular. However, the analytic models that are commonly applied in population health sciences limit the inference we are able to draw about the determination of health behaviour by factors, likely interrelated, across levels of influence. Complex system dynamic modelling techniques may be useful in enabling the adoption of a social epidemiologic approach in health behaviour and drug use research. We provide an example of a model that aims to incorporate factors at multiple levels of influence in understanding drug dependence. We conclude with suggestions about future directions in the field and how such models may serve as virtual laboratories for policy experiments aimed at improving health behaviour.

Implications of Inertia for Assessing Drug Control Policy: Why Upstream Interventions May Not Receive Due Credit

An empirically-derived model of national drug initiation is combined with a compartment model of trends in illicit drug use parameterized for Australia. Numbers of initiates seem to oscillate over time but with fluctuations whose aptitude diminishes over time. Lags and interpersonal variation in drug use careers smooth those oscillations for measures of problem use and total drug-related social cost, so even abrupt changes in initiation barely ripple the trajectory of drug-related social cost. Hence, the benefits of interventions that successfully disrupt initiation may be “hidden in plain sight” by being spread broadly over time so that they leave only the faintest finger-print detectable by before-and-after comparison. Thus, the absence of clear drops in drug-related problems does not imply that an upstream ntervention such as prevention or supply disruption did not produce substantial social benefits. More generally, drug use and related problems are often perceived of as stubborn, even intractable. Some of that perception stems from real limitations in the effectiveness of drug-control interventions. However, inertia inherent in the evolution of drug use can make even cost-effective interventions appear weak (and detrimental interventions appear benign).

Dynamic compartmental model of trends in Australian drug use

A five-state compartment model of trends in illicit drug use in Australia is parameterized using data from multiple sources. The model reproduces historical prevalence and supports what-if analyses under the assumption that past trajectories of drug escalation and desistance persist. For fixed initiation, the system has a unique stable equilibrium. The chief qualitative finding is that even though some users escalate rapidly, regular injection drug use still adjusts to changes in incidence with considerable inertia and delay. This has important policy implications, e.g., concerning the timing of reductions in drug-related social cost generated by interventions that reduce the social cost per injection user versus those that cut drug initiation.