In Reply

Risks Associated With the Non-Medicinal Use of Cannabis by Dr. rer. nat. Eva Hoch, Prof. Dr. med. Udo Bonnet, Prof. Dr. med. Rainer Thomasius, Dr. med. Florian Ganzer, Prof. Dr. med. Ursula Havemann-Reinecke and Prof. Dr. med. Ulrich W. Preuss in issue 16/2015

We thank Ulrich Braune and Dr Grotenhermen for their valuable suggestions and critical comments.

In our review article we aimed to summarize what is currently known about the potential risks of intensive leisure-time cannabis consumption (1). The European Monitoring Centre for Drugs and Drug Addiction refers to “intensive” consumers as persons who use cannabis on a daily or almost daily basis. This includes about 3 million persons in the European Union (2), not 12 million, as we incorrectly cited in our article. Only a few studies have investigated more specific cannabis use patterns in association with potential health-related or social problems. Distinguishing between “high” and “low” consumption, as suggested by Ulrich Braune, is, however, not enough to draw any conclusions about causation. First of all, these categories would have to be clearly defined and then consistently used in research. To this end, it would be necessary for experts internationally to agree on data collection standards. But even then the question of causality would remain unanswered. The retrospective twin study cited by Dr Grotenhermen (3) can ultimately neither rule out nor confirm whether psychological, somatic, or social problems are due to cannabis consumption or to other (genetic, environmental) factors. Prospective studies are needed to clarify this. So far, very few studies have comprehensively investigated persons from early childhood into adult age and have methodically controlled for a multitude of associated variables at the same time. One of the highest-quality studies is by Meier and colleagues (4). The authors conclude that starting to consume cannabis regularly early in life (before the 18th year of life) in particular has a detrimental effect in later cognitive performance.

When assessing the somatic sequelae of cannabis consumption, the influence of cannabis and its ingredients as well as the effects of associated variables have to be monitored closely. We therefore thank Dr Grotenhermen for mentioning a relevant study by Hashibe et al (5). It shows that, as in the studies we included in our article, tobacco consumption has to be considered in analyzing the effect of inhaled cannabis products. While THC by itself does not have a confirmed carcinogenic effect, cannabis smoke does contain a whole range of chemical compounds for which harmful effects can currently not be ruled out, as the author of the letter himself confirms with his own publications.

In conclusion, we would make a case for studying the risks of consumption of non-medical cannabis separately to the clinical data on medical cannabis consumption. The respective

target groups, expectations, consumption patterns, dosages, and settings are extremely different, and this has to be borne in mind.

DOI: 10.3238/arztebl.2016.0010

On behalf of the authors:

Dr. rer. nat. Eva Hoch

Klinik und Poliklinik für Psychiatrie und Psychotherapie

Klinikum der Universität München

Conflict of interest statement

The author declares that no conflict of interest exists.

References

1. Hoch E, Bonnet U, Thomasius R, Ganzer F, Havemann-Reinecke U, Preuss UW: Risks associated with the non-medicinal use of cannabis. Dtsch Arztebl Int 2015; 112: 271–8

2. European Monitoring Centre for Drugs and Drug Addiction: Treatment of cannabis-related disorders in Europe. Luxembourg: Publications Office of the European Union; 2015; EMCDDA Insights No 17.

3. Verweij KJ, Huizink AC, Agrawal A, Martin NG, Lynskey MT: Is the relationship between early-onset cannabis use and educational attainment causal or due to common liability? Drug Alcohol Depend 2013; 133: 580–6

4. Meier M, Avshalom C, Ambler A, et al.: Persistent cannabis users show neuropsychological decline from childhood to midlife. PNAS 2012; www.pnas.org/cgi/doi/10.1073/pnas.1206820109 (last accessed on 14 September 2015).

5. Hashibe M, Morgenstern H, Cui Y, et al.: Marijuana use and the risk of lung and upper aerodigestive tract cancers: results of a population-based case-control study. Cancer Epidemiol Biomarkers Prev 2006; 15: 1829–34