1. Introduction
According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), Substance Use Disorder (SUD) is characterized by a problematic pattern of substance use leading to clinically significant impairment or distress, manifested by cognitive, behavioral, and physiological symptoms [
1]. SUD rarely occurs in isolation and frequently coexists with other psychiatric disorders, including psychotic disorders, Attention-Deficit/Hyperactivity Disorder (ADHD), mood disorders, and personality disorders [
2,
3,
4]. This coexistence, currently conceptualized as Dual Disorder (DD), is associated with greater clinical severity, poorer psychosocial functioning, increased healthcare utilization, and more complex treatment needs than either condition alone [
5,
6,
7]. Consequently, understanding the factors that influence the clinical presentation and longitudinal evolution of DD has become a major focus of contemporary addiction psychiatry [
8,
9].
Among the psychiatric manifestations associated with SUD, psychotic symptoms represent one of the most clinically relevant complications [
9,
10,
11,
12]. The use of substances such as cannabis [
13,
14,
15], cocaine, and alcohol has been linked to hallucinations, delusions, and psychotic disorders, either through direct effects or by increasing vulnerability in predisposed individuals [
16]. Psychotic symptoms are associated with greater addiction severity, poorer psychosocial functioning, and worse clinical outcomes [
17,
18,
19]. However, their expression may vary according to age, gender [
20] and duration of substance use [
21], suggesting a dynamic interaction between addiction and psychosis throughout the lifespan [
20,
22,
23,
24].
Neurodevelopmental disorders also play an important role in the clinical heterogeneity of SUD. ADHD is highly prevalent among individuals with SUD and has been consistently associated with impulsivity, emotional dysregulation, violence, criminal behavior, and treatment difficulties [
25,
26,
27]. Although longitudinal studies indicate that hyperactivity and impulsivity tend to decrease with age, the extent to which these changes influence the clinical course of SUD remains insufficiently understood [
26,
27]. Similarly, growing evidence suggests a meaningful relationship between Autism Spectrum Disorder (ASD) traits and substance use. Nevertheless, the influence of age and chronic substance exposure on autistic traits in SUD populations has received limited attention [
28].
Impulsivity and violence are additional dimensions of particular relevance in DD [
29]. Both have been associated with poorer clinical outcomes, greater psychosocial impairment, and increased risk of relapse. Emotional dysregulation, a common feature across ADHD, SUD, and psychotic disorders, may contribute to aggressive behaviors and interpersonal conflict [
30,
31,
32]. Moreover, age appears to influence impulsivity and violent behavior, with younger individuals generally exhibiting higher levels of behavioral dyscontrol [
28].
Age and duration of substance use are key determinants of the clinical presentation and progression of SUD, influencing physical health, psychiatric symptomatology, cognitive functioning, and treatment trajectories [
33]. Previous research has independently associated psychotic symptoms, impulsivity, violence, ADHD, and ASD traits with SUD [
34,
35,
36]; however, the interaction of these dimensions across different stages of the lifespan remains insufficiently understood. Longitudinal studies simultaneously examining these factors in routine clinical settings are scarce. Therefore, the present study aimed to compare patients with prolonged substance use trajectories with those presenting shorter exposure histories, evaluating differences in psychotic symptoms, ADHD symptoms, autistic traits, impulsivity, violence, quality of life, and treatment retention over a six-month follow-up period.
We hypothesized that younger patients and those with shorter substance use trajectories would exhibit higher levels of psychotic symptoms, impulsivity, ADHD-related symptoms, and violence, whereas patients with prolonged substance use histories would demonstrate greater physical deterioration. Additionally, we hypothesized that greater psychiatric severity, particularly psychotic symptoms, impulsivity, and neurodevelopmental traits, would be associated with lower treatment retention during follow-up.
4. Discussion
Compared with the HL group, younger participants exhibited poorer scores for general health, vitality, social functioning, and emotional role. During follow-up, both groups showed a shift from physical to verbal aggression, consistent with the behavioural de-escalation models outlined in psychotherapy interventions for SUD. Younger participants also presented a higher prevalence of psychotic symptoms and greater impulsivity than older individuals. In contrast, treatment retention appeared to be more closely associated with overall physical functioning than with violence or impulsivity, highlighting the potential value of age- and severity-specific management strategies in patients with SUD.
Compared with the HL group, the sL group showed poorer scores for general health, vitality, social functioning, and emotional role, with no significant improvement after six months of follow-up. Several explanations may account for these findings. First, the lower representation of severe cases among older participants may reflect barriers to treatment associated with physical or functional deterioration. Second, survivor bias may contribute to these results, as previous studies have shown that individuals with more severe SUD trajectories have higher mortality rates or require more intensive levels of care [
46,
47]. Finally, younger individuals seeking treatment for the first time may present greater clinical impairment associated with more frequent and complex substance use patterns [
48]. Taken together, these findings highlight the importance of intervention strategies aimed not only at reducing substance use but also at preserving physical functioning throughout the course of the disorder.
Younger individuals exhibited greater difficulty controlling violent behaviours. One of the main findings of this study was that, following treatment, both groups showed a reduction in physical aggression accompanied by an increase in verbal aggression. Although previous studies have reported inconsistent findings, there is evidence that treatment is more effective in reducing physical than verbal violence [
49,
50]. One possible explanation is that physical aggression is more closely related to impulsivity and emotional dysregulation, whereas verbal aggression may reflect more deliberate forms of interpersonal communication [
51]. The reduction in physical aggression may therefore be associated with improvements in emotional regulation during treatment, while the increase in verbal aggression could represent a displacement from physical to verbal expression. Consequently, verbal aggression may require additional therapeutic approaches, particularly cognitive-behavioural interventions targeting anger management and interpersonal communication [
52].
A possible explanation is that psychological interventions may partially compensate for neurobiological alterations associated with long-term substance use. Chronic exposure to psychoactive substances has been linked to dysfunction in monoaminergic systems involved in emotional regulation and impulse control [
52,
53]. Although the present study did not assess neurobiological markers, the reduction in physical aggression suggests that treatment may improve behavioral regulation even when underlying neurobiological vulnerabilities persist. Thus, psychological interventions targeting emotional regulation may remain beneficial across different stages of addiction.
In contrast to previous studies reporting lower treatment adherence among patients with greater violence and clinical instability [
53], the most violent individuals in our cohort were more likely to remain in treatment. Although the mechanisms underlying this finding remain uncertain, violence may represent a marker of greater clinical severity and reinforce the need to incorporate anger management and violence-focused interventions into SUD treatment. At baseline, younger participants exhibited greater difficulties in decision-making and higher overall impulsivity, whereas older individuals or those with prolonged substance use histories showed higher motor impulsivity. Previous studies have associated elevated impulsivity with poorer treatment adherence [
54]; therefore, early retention strategies may be particularly important for patients with marked impulsive traits, given the well-established benefits of sustained treatment engagement on long-term outcomes [
55,
56]. Furthermore, impulsivity appeared to remain elevated among participants with ADHD who continued treatment, particularly in the HL group. This finding suggests that prolonged substance use may be associated with persistent ADHD symptomatology, highlighting the need for targeted interventions addressing impulsivity in this population.
In the HL group, the reduction in impulsivity and hyperactivity suggests that these symptoms may decline with age. Among adolescent samples, impulsivity has been characterized by a preference for immediate, lower-value rewards over larger, delayed rewards [
57]. This, in the long term, may influence the initiation and persistence of substance use. Although impulsivity has been extensively investigated in adolescents and young adults, evidence in older individuals with SUD remains limited. Several mechanisms may explain these findings. First, longitudinal studies have shown that hyperactive-impulsive symptoms of ADHD tend to decrease with age [
57,
58]. Second, survivor bias may contribute to this pattern, as individuals with greater impulsivity and more severe comorbidities, including ADHD and SUD, have a reduced life expectancy [
20,
59]. Indeed, previous longitudinal studies have reported increased mortality among individuals with the hyperactive-impulsive presentation of ADHD, particularly from causes related to impulsive behaviours [
26]. Finally, age-related differences in the psychometric properties of the ASRS have also been described [
58]. Taken together, these findings suggest that the relationship between age, impulsivity, and SUD is multifactorial and should be interpreted within a lifespan perspective.
A notable finding was the high prevalence of ASD traits among individuals with SUD, affecting 45.7% of the HL group and 54.3% of the sL group, with no significant between-group differences. ASD traits may influence substance use patterns, treatment response, and the management of violent behaviours, particularly among younger individuals. One possible explanation is the self-medication hypothesis [
60], whereby substance use serves as a coping strategy for social and emotional difficulties. These findings support the need to adapt SUD treatment to the specific needs of this population [
61]. Differences between groups were observed only for the imagination subscale, which reflects cognitive flexibility and mental simulation abilities. This finding may be related to age-associated reductions in cognitive flexibility [
54] and/or the neurotoxic effects of prolonged substance use on brain regions involved in executive functioning, including the hippocampus, prefrontal cortex, and frontotemporal networks [
62,
63]. Although ASD traits were not associated with treatment retention, their presence may complicate the clinical management of SUD because of their frequent overlap with ADHD and psychotic symptoms [
64].
The present study confirmed the high prevalence of psychotic symptoms among individuals with SUD, consistent with previous reports [
18,
65]. However, psychotic symptoms were less prevalent among older participants. This difference may be related to the higher prevalence of cocaine use in the younger group, as cocaine has been consistently associated with an increased risk of psychotic episodes, and because the peak incidence of psychosis typically occurs during the second and third decades of life [
66]. Younger participants also exhibited higher levels of impulsivity, supporting previous evidence of an association between impulsivity and psychotic symptoms in individuals with [
18] and without SUD [
67]. Although earlier studies linked psychotic symptoms and impulsivity to violent behaviour among stimulant users [
68], no such association was observed in the present cohort. This discrepancy may reflect the absence of methamphetamine users, the substance most strongly associated with violence in previous reports. Nevertheless, the early identification and management of psychotic symptoms remain clinically relevant, given their association with greater addiction severity [
69].
No consistent pattern of factors associated with treatment retention was identified, consistent with previous studies reporting that treatment retention is not consistently predicted by violence, impulsivity, psychotic symptoms, or ADHD [
70,
71]. Survivor bias should also be considered when interpreting these findings, as individuals with long-standing dual disorders and greater functional impairment are more likely to require residential or long-term care, thereby reducing their representation in outpatient cohorts [
72]. This interpretation is further supported by the elevated mortality rates reported among individuals with SUD and psychotic disorders, resulting from both medical causes and suicide [
73,
74,
75]. Consequently, the apparent reduction in the prevalence of some clinical conditions over time may reflect not only genuine clinical improvement but also the selective loss of participants with poorer health, who are more likely to discontinue treatment, miss follow-up assessments, transition to higher-intensity services, relocate, or die prematurely. As a result, patients remaining in outpatient follow-up may represent a relatively more functional subgroup despite persisting impulsivity, interpersonal conflict, or psychiatric symptoms, including psychotic symptoms and ASD traits. This mechanism may also partly explain the lower clinical severity observed among older participants.
The present study has several limitations. First, the use of a non-probabilistic sample and recruitment from a single outpatient center may limit the generalizability of the findings to other SUD populations. Second, given the high prevalence of polysubstance use, years of use were analyzed separately for each substance rather than as integrated patterns of combined use, which may have provided a more comprehensive characterization of clinical profiles [
76]. Third, attrition during follow-up reduced the sample available for longitudinal analyses, a common challenge in SUD research [
77,
78]. Treatment retention was operationalized as completion of the 6-month follow-up assessment; however, because reasons for missing follow-up evaluations were not systematically recorded, non-retention may reflect different clinical or administrative circumstances. Additionally, the detailed dropout-management discussion and sensitivity analyses suggested by the reviewer were considered beyond the scope of the manuscript and therefore are not presented in the article, although they were performed internally for review. Finally, neurobiological measures, such as neuroimaging, were not included, and individuals with schizophrenia or bipolar disorder with psychotic features were excluded. Despite these limitations, the study was conducted in a homogeneous outpatient clinical sample, supporting the applicability of the findings to routine clinical practice.
In conclusion, younger individuals with SUD exhibited a higher prevalence of psychotic symptoms, greater impulsivity, and more violence-related behaviours than older participants or those with prolonged substance use histories. The observed shift from physical to verbal aggression during follow-up highlights the potential value of psychological interventions targeting emotional regulation and interpersonal functioning. However, it is also possible that discontinuing use may temporarily exacerbate symptoms, as the potential self-medication effect diminishes. The high prevalence of ADHD- and ASD-related traits further supports the importance of routinely assessing neurodevelopmental characteristics in patients with SUD. Although several clinical differences were observed according to age and duration of substance use, the mechanisms underlying these associations remain uncertain. Given the observational design of the study, these findings should be interpreted as associations rather than causal relationships. Overall, the results support the implementation of age-sensitive assessment and treatment strategies that prioritize treatment retention, early identification of psychotic symptoms and neurodevelopmental traits, and interventions tailored to the clinical profile of each patient.
Future studies should include larger, multicenter cohorts and incorporate neurobiological measures, such as neuroimaging, to further investigate the mechanisms underlying the associations observed in this study. In addition, larger samples would allow comparisons of longitudinal outcomes across specific substance use disorder subgroups, thereby improving the understanding of how different patterns of substance use influence clinical presentation, treatment retention, and long-term outcomes.