Change in Cognitive Function in Stimulant Users

In Hong Kong, methamphetamine use is common and cocaine use has increased steadily over the past few years. While the use of ketamine decreased from 35.8% in 2015 to 13.9% in 2017, methamphetamine and cocaine have become the most commonly used psychotropic substances and account for more than 50% of drug abuses cases in 2017. Among all stimulants, methamphetamine is most commonly used because it releases three times more dopamine than cocaine and the effects can last from eight to twelve hours, compared to two hours for cocaine. On top of its extreme effects, methamphetamine is relatively inexpensive, making it even more accessible to the young population.

Misuse of methamphetamine has long been associated with profound psychological and cognitive disturbance. In reviewing the cognitive data from reasonably well-matched groups of chronic methamphetamine users and healthy controls, the majority of studies have found that chronic methamphetamine users had lower scores on at least some cognitive tests, although some studies are exceptions with entirely nonsignificant differences. A meta-analysis of 17 cross-sectional studies found that chronic methamphetamine users demonstrated significantly lower cognitive scores than healthy controls. The effects were largest for measures of learning, executive functions, memory, and processing speed, although the majority of cognitive domains significantly differed between the groups.

Concerns has been emerging regarding the methodology of the aforementioned results. In particular, the appropriateness of using healthy controls to examine the cognitive effects of stimulant use has been questioned. Much of the published research has fallen victim to using controls with significant baseline differences from the chronic stimulant users, such as years of education. In addition, none of the studies available provided scatter plots of their cognitive data to evaluate the overlap in performance between chronic stimulant users and healthy controls. In fact, many chronic stimulant users have normal cognitive function when compared with normative data. Therefore, the use of the term 'impairment' or 'deficit' in many studies is not fully justified. Another limitation is that it cannot differentiate cognitive weaknesses that may predate stimulant use from those that result from it. Notably, longitudinal studies have shown that childhood deficits in executive function can predict drug abuse in adolescence, suggesting that at least some of the cognitive weaknesses pre-exist in chronic stimulant user. These and other limitations provoked a conclusion that the evidence for cognitive deficits in chronic stimulant users is weak.

In order to overcome the methodological issues observed in previous cross-sectional studies, we propose to conduct a prospective studies to determine the change in cognitive function among stimulant users over time.