This special issue of European Addiction Research (EAR) ratifies that there is an urgent need to foreground the initially ignored and then underestimated topic of an addiction-ADHD comorbidity, and to acknowledge the 10th anniversary of the International Collaboration on ADHD and Substance Abuse (ICASA: https://www.adhdandsubstanceabuse.org) that has dedicated itself to that unmet need.

ADHD and substance use disorders (SUDs) are both common mental disorders that commonly co-occur [1]. In the general population, ADHD occurs in about 6% of children and adolescents and about 2.5% of adults [2, 3]. Combining 27 general population and clinical studies with more than 4,000 ADHD participants and almost 7,000 non-ADHD participants, those with ADHD were about 2–3 times more likely to develop a nicotine, alcohol, marijuana, cocaine, and/or other substance use disorders than those without ADHD [4]. In adult SUD patients, the prevalence of ADHD is estimated at around 20% [5-7]. Furthermore, SUD patients with comorbid ADHD have an earlier onset of substance use, a faster transition to SUD, and a higher risk of relapse compared to those without ADHD [8-11].

Given the high prevalence and impact of co-occurring ADHD in patients with SUD, this is an important clinical challenge that calls for scientific evidence supporting clinical decision making. To this end, ICASA was founded 10 years ago. Over the past decade, ICASA and others have built a great deal of evidence concerning SUD-ADHD comorbidity. Where ADHD is still often overlooked or ignored in SUD patients, these scientific insights indicate that in patients with SUD, ADHD should be considered a valid and highly relevant psychiatric diagnosis. It is a chronic neurodevelopmental condition, where symptoms of inattention and/or hyperactivity/impulsivity lead to additional impairments in many aspects of living.

Given the high prevalence and impact of ADHD in patients with SUD, screening for ADHD should be part of standard intake procedures in addiction care. Moreover, ADHD can be reliably assessed in patients with SUD even when they are not (yet) fully abstinent [12]. There are several reliable screening and diagnostic tools available for this purpose, as summarized in the ICASA consensus statement on the screening, diagnosis, and treatment of adult SUD patients with comorbid ADHD [13]. In addition, several studies suggest that early stimulant treatment of childhood ADHD can prevent the development of SUD [10, 14-21] and that pharmacotherapy of ADHD can improve outcomes in SUD patients with comorbid ADHD [13, 22]. It should be noted, however, that studies have also shown that the neural response to stimulant treatment in ADHD patients with SUD may differ from the response in ADHD patients without SUD [23] and that – as a consequence – higher doses may be needed in at least some of the patients with ADHD and SUD [24, 25]. These insights are highly relevant for clinical practice and should find their way to (inter)national guidelines.

In addition, there are new insights in the biological underpinnings of the frequent co-occurrence of SUD and ADHD. ADHD and SUD belong to the more heritable psychiatric conditions [26-31], and both twin and genome-wide association studies have indicated a strong shared heritability of ADHD and SUD [32, 33]. These findings suggest that the frequent co-occurrence of these conditions has a shared biological background. Similarly, neurocognitive and neuroimaging studies have identified cognitive control pathways, reward pathways, and emotion regulation pathways to be centrally involved in both conditions [34, 35]. Though these insights further our understanding of this frequent comorbidity, there is still much to learn about the exact causal mechanisms contributing to SUD-ADHD comorbidity.

The 10-year anniversary of ICASA marks a decade of intense and fruitful international collaboration. With this special issue of EAR, we share some of the most recent developments in this field of research with special attention to some new findings from the ICASA network. This special issue contains 6 papers on recently collected data by the ICASA network showing increased severity of SUD and increased severity of smoking in SUD patients with comorbid ADHD compared to those without ADHD (Icick et al., 2020; Sanchez-Garcia et al., 2020, respectively); prevalence data on ADHD in prisoners with SUD (Velez-Patrana et al., 2020); new data on the role of antisocial traits in the development of comorbidity between ADHD and SUD (Gonzalez et al., 2020); a review on the role of sustained attention in the diagnosis of patients with SUD-ADHD comorbidity (Slobodin, 2020); and finally a consensus paper concerning the clinical assessment and treatment of adolescents with SUD and ADHD (Özgen et al., 2020). Here we like to emphasize that the ICASA consensus papers (Ozgen et al., 2020; [13]), although important and clinically needed, are no substitution for evidence-based guidelines.

Additionally, several papers are included in this issue focusing on frequent other comorbidities in patients with SUD and ADHD, including PTSD (Luderer et al., 2020), personality traits (Moggi et al., 2020), polysubstance use in SUD patients with and without ADHD (Lugoboni et al., 2020), and the effect of alcohol use and a family history of alcohol dependence on reward-delay aversion in adolescents with ADHD with and without SUDs (Paraskevopoulou et al., 2020).

Based on data presented in this special issue of EAR, we conclude that this field has shown great advancement over the past years, but also that a lot of work has yet to be done. For instance, future studies should address the question concerning personalized treatment of patients with both conditions. For instance, what is the best pharmacological treatment of patients with SUD and ADHD (e.g., stimulants or atomoxetine) and how can robust doses of stimulants be prescribed without the risk of serious side-effects (insomnia, cardiovascular problems, and diversion)? Furthermore, there is a remarkable paucity of data on (integrated) psychological treatment of SUD with comorbid ADHD and the outcome predictors of such interventions [36, 37]. There is an urgent need for evidence concerning which psychological treatments work best for patients suffering both conditions, and how SUD treatment should be adapted when patients have comorbid ADHD, and vice versa. Another major issue in the existing literature is the lack of prospective data on real-life clinical experiences in the treatment of SUD patients with comorbid ADHD. The INCAS study, as presented in this issue (Brynte et al., 2020), will shed light on the course of these disorders when these patients enter addiction care, and provide insight into outcome of different treatment approaches across the participating countries.

Taken together, the SUD-ADHD field is rapidly developing. After 10 years of work, the ICASA group is alive and kicking. New members have been welcomed over the recent years, and several young scientists have joined, dedicating their work to this relevant topic. ICASA strives to foster its work in prevention and clinical care for patients with SUD and comorbid ADHD through scientific evidence. We wish all readers an inspiring experience when reading this Special Issue.

None of the authors have a conflict of interest to declare.

1.
Wilens
TE
,
Morrison
NR
.
The intersection of attention-deficit/hyperactivity disorder and substance abuse
.
Curr Opin Psychiatry
.
2011
;
24
(
4
):
280
5
.
2.
Simon
V
,
Czobor
P
,
Bálint
S
,
Mészáros
A
,
Bitter
I
.
Prevalence and correlates of adult attention-deficit hyperactivity disorder: meta-analysis
.
Br J Psychiatry
.
2009
;
194
(
3
):
204
11
.
3.
Willcutt
EG
.
The prevalence of DSM-IV attention-deficit/hyperactivity disorder: a meta-analytic review
.
Neurotherapeutics
.
2012
;
9
(
3
):
490
9
.
4.
Lee
SS
,
Humphreys
KL
,
Flory
K
,
Liu
R
,
Glass
K
.
Prospective association of childhood attention-deficit/hyperactivity disorder (ADHD) and substance use and abuse/dependence: a meta-analytic review
.
Clin Psychol Rev
.
2011
;
31
(
3
):
328
41
.
5.
van Emmerik-van Oortmerssen
K
,
van de Glind
G
,
Koeter
MW
,
Allsop
S
,
Auriacombe
M
,
Barta
C
, et al.
Psychiatric comorbidity in treatment-seeking substance use disorder patients with and without attention deficit hyperactivity disorder: results of the IASP study
.
Addiction
.
2014
;
109
(
2
):
262
72
.
6.
van de Glind
G
,
Konstenius
M
,
Koeter
MWJ
,
van Emmerik-van Oortmerssen
K
,
Carpentier
PJ
,
Kaye
S
, et al.
Variability in the prevalence of adult ADHD in treatment seeking substance use disorder patients: results from an international multi-center study exploring DSM-IV and DSM-5 criteria
.
Drug Alcohol Depend
.
2014
;
134
:
158
66
.
7.
Groenman
AP
,
Oosterlaan
J
,
Rommelse
N
,
Franke
B
,
Roeyers
H
,
Oades
RD
, et al.
Substance use disorders in adolescents with attention deficit hyperactivity disorder: a 4-year follow-up study
.
Addiction
.
2013
;
108
(
8
):
1503
11
.
8.
Hahesy
AL
,
Wilens
TE
,
Biederman
J
,
Van Patten
SL
,
Spencer
T
.
Temporal association between childhood psychopathology and substance use disorders: findings from a sample of adults with opioid or alcohol dependency
.
Psychiatry Res
.
2002
;
109
(
3
):
245
53
.
9.
McCabe
SE
,
Dickinson
K
,
West
BT
,
Wilens
TE
.
Age of onset, duration, and type of medication therapy for attention-deficit/hyperactivity disorder and substance use during adolescence: a multi-cohort national study
.
J Am Acad Child Adolesc Psychiatry
.
2016
;
55
(
6
):
479
86
.
10.
Wilens
TE
.
Impact of ADHD and its treatment on substance abuse in adults
.
J Clin Psychiatry
.
2004
;
65
(
Suppl 3
):
38
45
.
11.
Castells
X
,
Ramos-Quiroga
JA
,
Rigau
D
,
Bosch
R
,
Nogueira
M
,
Vidal
X
, et al.
Efficacy of methylphenidate for adults with attention-deficit hyperactivity disorder: a meta-regression analysis
.
CNS Drugs
.
2011
;
25
(
2
):
157
69
.
12.
van Emmerik-van Oortmerssen
K
,
Vedel
E
,
Kramer
FJ
,
Koeter
MW
,
Schoevers
RA
,
van den Brink
W
.
Diagnosing ADHD during active substance use: feasible or flawed?
Drug Alcohol Depend
.
2017
;
180
:
371
5
.
13.
Crunelle
CL
,
van den Brink
W
,
Moggi
F
,
Konstenius
M
,
Franck
J
,
Levin
FR
, et al.
International consensus statement on screening, diagnosis and treatment of substance use disorder patients with comorbid attention deficit/hyperactivity disorder
.
Eur Addict Res
.
2018
;
24
(
1
):
43
51
.
14.
Biederman
J
,
Monuteaux
MC
,
Spencer
T
,
Wilens
TE
,
Macpherson
HA
,
Faraone
SV
.
Stimulant therapy and risk for subsequent substance use disorders in male adults with ADHD: a naturalistic controlled 10-year follow-up study
.
Am J Psychiatry
.
2008
;
165
(
5
):
597
603
.
15.
Faraone
SV
,
Wilens
T
.
Does stimulant treatment lead to substance use disorders?
J Clin Psychiatry
.
2003
;
64
(
Suppl 11
):
9
13
.
16.
Faraone
SV
,
Biederman
J
,
Wilens
TE
,
Adamson
J
.
A naturalistic study of the effects of pharmacotherapy on substance use disorders among ADHD adults
.
Psychol Med
.
2007
;
37
(
12
):
1743
52
.
17.
Faraone
SV
,
Wilens
TE
.
Effect of stimulant medications for attention-deficit/hyperactivity disorder on later substance use and the potential for stimulant misuse, abuse, and diversion
.
J Clin Psychiatry
.
2007
;
68
(
Suppl 11
):
15
22
.
18.
Groenman
AP
,
Oosterlaan
J
,
Rommelse
NN
,
Franke
B
,
Greven
CU
,
Hoekstra
PJ
, et al.
Stimulant treatment for attention-deficit hyperactivity disorder and risk of developing substance use disorder
.
Br J Psychiatry
.
2013
;
203
(
2
):
112
9
.
19.
Groenman
AP
,
Schweren
LJS
,
Weeda
W
,
Luman
M
,
Noordermeer
SDS
,
Heslenfeld
DJ
, et al.
Stimulant treatment profiles predicting co-occurring substance use disorders in individuals with attention-deficit/hyperactivity disorder
.
Eur Child Adolesc Psychiatry
.
2019
;
28
(
9
):
1213
22
.
20.
Wilens
TE
,
Faraone
SV
,
Biederman
J
,
Gunawardene
S
.
Does stimulant therapy of attention-deficit/hyperactivity disorder beget later substance abuse? A meta-analytic review of the literature
.
Pediatrics
.
2003
;
111
(
1
):
179
85
.
21.
Wilens
TE
,
Adamson
J
,
Monuteaux
MC
,
Faraone
SV
,
Schillinger
M
,
Westerberg
D
, et al.
Effect of prior stimulant treatment for attention-deficit/hyperactivity disorder on subsequent risk for cigarette smoking and alcohol and drug use disorders in adolescents
.
Arch Pediatr Adolesc Med
.
2008
;
162
(
10
):
916
21
.
22.
Boland
H
,
DiSalvo
M
,
Fried
R
,
Woodworth
KY
,
Wilens
T
,
Faraone
SV
, et al.
A literature review and meta-analysis on the effects of ADHD medications on functional outcomes
.
J Psychiatr Res
.
2020
;
123
:
21
30
.
23.
Crunelle
CL
,
van den Brink
W
,
Veltman
DJ
,
van Emmerik-van Oortmerssen
K
,
Dom
G
,
Schoevers
RA
, et al.
Low dopamine transporter occupancy by methylphenidate as a possible reason for reduced treatment effectiveness in ADHD patients with cocaine dependence
.
Eur Neuropsychopharmacol
.
2013
;
23
(
12
):
1714
23
.
24.
Konstenius
M
,
Jayaram-Lindström
N
,
Guterstam
J
,
Beck
O
,
Philips
B
,
Franck
J
.
Methylphenidate for attention deficit hyperactivity disorder and drug relapse in criminal offenders with substance dependence: a 24-week randomized placebo-controlled trial
.
Addiction
.
2014
;
109
(
3
):
440
9
.
25.
Levin
FR
,
Mariani
JJ
,
Specker
S
,
Mooney
M
,
Mahony
A
,
Brooks
DJ
, et al.
Extended-release mixed amphetamine salts vs placebo for comorbid adult attention-deficit/hyperactivity disorder and cocaine use disorder: a randomized clinical trial
.
JAMA Psychiatry
.
2015
;
72
(
6
):
593
602
.
26.
Bidwell
LC
,
Gray
JC
,
Weafer
J
,
Palmer
AA
,
de Wit
H
,
MacKillop
J
.
Genetic influences on ADHD symptom dimensions: examination of a priori candidates, gene-based tests, genome-wide variation, and SNP heritability
.
Am J Med Genet B Neuropsychiatr Genet
.
2017
;
174
(
4
):
458
66
.
27.
Clarke
TK
,
Adams
MJ
,
Davies
G
,
Howard
DM
,
Hall
LS
,
Padmanabhan
S
, et al.
Genome-wide association study of alcohol consumption and genetic overlap with other health-related traits in UK Biobank (N = 112,117)
.
Mol Psychiatry
.
2017
;
22
(
10
):
1376
84
.
28.
Demers
CH
,
Bogdan
R
,
Agrawal
A
.
The genetics, neurogenetics and pharmacogenetics of addiction
.
Curr Behav Neurosci Rep
.
2014
;
1
(
1
):
33
44
.
29.
Faraone
SV
,
Larsson
H
.
Genetics of attention deficit hyperactivity disorder
.
Mol Psychiatry
.
2019
;
24
(
4
):
562
75
.
30.
Gorwood
P
,
Le Strat
Y
,
Ramoz
N
.
Genetics of addictive behavior: the example of nicotine dependence
.
Dialogues Clin Neurosci
.
2017
;
19
(
3
):
237
45
.
31.
Wang
JC
,
Kapoor
M
,
Goate
AM
.
The genetics of substance dependence
.
Annu Rev Genomics Hum Genet
.
2012
;
13
:
241
61
.
32.
Vink
J
,
Schellekens
A
.
Relating addiction and psychiatric disorders
.
Science
.
2018
;
361
(
6409
):
1323
4
.
33.
Derks
EM
,
Vink
JM
,
Willemsen
G
,
van den Brink
W
,
Boomsma
DI
.
Genetic and environmental influences on the relationship between adult ADHD symptoms and self-reported problem drinking in 6024 Dutch twins
.
Psychol Med
.
2014
;
44
(
12
):
2673
83
.
34.
Luijten
M
,
Schellekens
AF
,
Kuehn
S
,
Machielse
MWJ
,
Sescousse
G
.
Disruption of reward processing in addiction an image-based meta-analysis of functional magnetic resonance imaging studies
.
Jama Psychiatry
.
2017
;
74
(
4
):
387
98
.
35.
Plichta
MM
,
Scheres
A
.
Ventral-striatal responsiveness during reward anticipation in ADHD and its relation to trait impulsivity in the healthy population: a meta-analytic review of the fMRI literature
.
Neurosci Biobehav Rev
.
2014
;
38
:
125
34
.
36.
van Emmerik-van Oortmerssen
K
,
Vedel
E
,
Kramer
FJ
,
Blankers
M
,
Dekker
JJM
,
van den Brink
W
, et al.
Integrated cognitive behavioral therapy for ADHD in adult substance use disorder patients: results of a randomized clinical trial
.
Drug Alcohol Depend
.
2019
;
197
:
28
3
37.
van Emmerik-van Oortmerssen
K
,
Blankers
M
,
Vedel
E
,
Kramer
F
,
Goudriaan
AE
,
van den Brink
W
, et al.
Prediction of drop-out and outcome in integrated cognitive behavioral therapy for ADHD and SUD: results from a randomized clinical trial
.
Addict Behav
.
2020
;
103
:
106228
.
Open Access License / Drug Dosage / Disclaimer
This article is licensed under the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC). Usage and distribution for commercial purposes requires written permission. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.