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Authors’ reply
Annabeth P. Groenman, Jaap Oosterlaan, Nanda N. J. Rommelse, Barbara
Franke, Corina U. Greven, Pieter J. Hoekstra, Catharina A. Hartman,
Marjolein Luman, Herbert Roeyers, Robert D. Oades, Joseph A. Sergeant,
Jan K. Buitelaar, Stephen V. Faraone
The British Journal of Psychiatry Jun 2014, 204 (6) 490-491; DOI: 10.1192/bjp.204.6.490b
In their letter, Verma and colleagues make the interesting point that
possibly the age at first stimulant use × current age interaction
effect found in our paper might be influenced by our selection of patients.
Including individuals with stimulant treatment duration longer than
12 months in our analyses, we found a protective effect of earlier age
at first stimulant use on the development of substance use disorder
(odds ratio (OR) = 0.95, Wald Chi-2 = 13.78, P<0.001). Verma et al
are concerned that we excluded patients with shorter treatment durations.
However, when we include all individuals who ever used stimulants, we
find the same effect (OR = 0.95, Wald Chi-2 = 11.89, P = 0.001).
Purely for illustrative purposes, we plotted the predicted probability
of substance use disorder for the control group in Fig. 1. The figure
shows that delay in the first age at stimulant use leads to marked increases
in the probability of developing substance use disorder. In our article,
we examined whether the effect of stimulant treatment depended on other
factors (i.e. current use of stimulants, age at stimulant treatment
initiation, age-adjusted duration of stimulant use and age-adjusted
cumulative dosage), but found no other significant predictors than age
at first stimulant use.
http://d34jb20qqe27k2.cloudfront.net/content/bjprcpsych/204/6/490.3/F1.large.jpg?width=800&height=600&carousel=1
Fig. 1 Predicted probability of substance use disorders (SUDs).
Purely for illustrative purposes, we plotted the predicted probability
of substance use disorder according to a generalised estimated equation
model, that included age, gender and the interaction between age and
age at first stimulant use. For healthy controls the model only includes
age and gender. Below average age at first stimulant use: participants
started before age 8.1 years; above average age at first stimulant use:
participants started after age 8.1 years. Please note that predictions
for healthy controls are not the product of the same model as prediction
for stimulant groups.
Verma and colleagues refer to a meta-analysis, but provide the wrong
citation. Recently, a meta-analysis on this topic was published. Here
no difference was found between treated and untreated patients in risk
of developing substance use disorder (including alcohol, marijuana,
cocaine and non-specific drugs) and nicotine use. Unfortunately, in
this meta-analysis specific moderator variables such as age at first
stimulant use were not taken into account, probably because of the relatively
low numbers of studies to date that include such variables.
We thank the authors for discovering the mistake in the table, 9% should
have read 59%.
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