Early Onset Cannabis Use and Progression to other Drug Use in a Sample of Dutch Twins

Abstract

One possible explanation of the commonly reported associations between early onset cannabis use and elevated risks of other illicit drug use is that early onset cannabis use increases access and availability to other drugs. It was this argument that in part motivated policy changes in the Netherlands that led to the de facto legalization of cannabis there. This study examines, using a co-twin control design, whether previously observed associ-ations between early onset cannabis use and elevated lifetime rates of other illicit drug use would also be observed in a sample of 219 same sex Dutch twin pairs discordant for cannabis use before age 18. After adjustment for covariates, rates of lifetime party drug use (OR=7.4, 95% CI=2.3-23.4), hard drug use (OR=16.5, 95% CI=2.4-111.3), but not regular cannabis use (OR=1.3, 95% CI=0.3-5.1) were significantly elevated in individuals who reported early onset cannabis use, relative to their co-twin who had not used cannabis by age 18. The elevated odds of subsequent illicit drug use in early cannabis users relative to their non early using cotwins suggests that this association could not be explained by common familial risk factors, either genetic or environmental, for which our co-twin methodology provided rigorous control.

Mental Health and Cannabis Abusers of Kashmir Valley

“…This relationship persisted after controlling for non-shared environmental factors [21]. Similar results have been reported in discordant twin studies in the USA [22] and Netherlands [23]. The psychotic symptoms or psychotic disorders were seen higher in regular cannabis users than in non-users [24].…”

The Genetics of Cannabis Involvement in Humans: A Genetic Epidemiological Perspective

“…genetic and environmental risk factors that influence the liability to use of any drug, but the 'gateway' model cannot be excluded. A Dutch twin study published while the present paper was in press addresses this question specifically [6]. Originally the legalization of cannabis in the Netherlands was prompted by the desire to break the link between cannabis and hard drugs, as 'coffee-shops' should prevent young people coming into contact with hard drug users.…”

“…In addition, cannabis use itself may serve as a psychosocial environmental moderator of the genetic vulnerability to other psychopathology [4,5]. Genetically informative studies tend to focus on the interaction between cannabis use and genetic vulnerability to other illicit drug use [6]. A more pressing concern is that cannabis users may also be at increased risk for transitioning from experimentation to regular use, as well as from regular use to dependence of commonly available, 'legal' and highly addictive substances such as nicotine and alcohol [7][8][9].…”

What Can We Learn from the Dutch Cannabis Coffeeshop Experience?

“…There are several other lines of indirect evidence. Concordance rates for marijuana use by twins in the Netherlands and Australia are similar, which could suggest a common developmental pathway irrespective of law or culture (Lynskey, Vink, & Boomsma, 2006). Using a bivariate duration model, van Ours (2003) -a labor economist who studies "stepping stones" in other economic contexts --finds evidence consistent with a causal gateway effect in Amsterdam, but he concludes that:…”