The current study aimed to understand the impact of the COVID-19 pandemic on (i) patterns of AOD use among clients who sought treatment for a SUD during the pandemic, (ii) mental health associates of changes in AOD use and (iii) changes in the contextual factors associated with AOD use. In a sample of recent treatment-seeking people, quantities of use increased for tobacco and cannabis following the pandemic. Self-reported frequency of cannabis use and other illicit drugs increased for the majority of these individuals, while frequency and quantity of methamphetamines decreased. Depression, anxiety and resilience were associated with changes in frequency of AOD use following the pandemic. Compared to AOD use before the pandemic, greater depression was associated with an increased frequency of alcohol use among clients, and using AOD more frequently for enjoyment and loneliness. Greater anxiety was associated with using cannabis less frequently. Regarding other substances, greater anxiety was also associated with using AOD earlier in the day and using for boredom more frequently following the pandemic. Resilience was most strongly linked with no change in the frequency of cannabis and methamphetamine use.

Retrospective estimates of AOD quantities used now compared to pre-pandemic were significantly increased for tobacco and cannabis, but estimates for alcohol and methamphetamines did not change. This partly supports prior reports that tobacco ³³ and cannabis use^15,33 use have increased following the pandemic. The lack of change in quantities of methamphetamine use may reflect market factors and the availability of the drug, where decreased availability has been cited as a common reason for using less. ¹⁷ The lack of change in the reported quantity of alcohol use in the current study does not concur with existing reports,^8,14,15 however these previous studies are not within treatment-seeking populations. Prior researchers have proposed declines in AOD use following the pandemic in community and non-dependent samples, which are attributable to restricted social settings,^16,34 compared with exacerbation of AOD use in dependent groups due to greater stress, financial pressure and uncertainty. ³⁵ The current findings partly support this; however, alcohol use did not increase. This may suggest that alcohol use in treatment-seeking groups may differ to other non-treatment seeking substance-using populations, yet the non-probabilistic sample and small response rate in the current study limits generalisation to all people seeking treatment. It is also important to note that reported quantities of use in the current study were retrospective and recalled from 4 to 6 months prior to completing the survey, the accuracy of the pre-pandemic quantities may be susceptible to recall biases. Longitudinal assessments of AOD quantity that emerge following the pandemic will enable more definitive claims of individual change in AOD use among individuals in treatment for SUDs.

Reported changes in the perceived frequency and quantity of AOD use among the current participants following the pandemic differed across substances, and a subset of individuals reported using AODs more frequently and in higher quantities. The current findings indicated this may be attributable to different mental health risk factors and resilience. The association between depression and using alcohol more frequently indicates that low mood may be a risk factor for participants with alcohol use disorder following social distancing measures, including using alcohol to increase pleasure and cope with loneliness. Depression and loneliness during the pandemic have been argued to exert an impact on mental health and functioning,^36,37 with heightened concerns for suicide risk in psychiatric and substance-using populations. ³⁸ Loneliness has been correlated with depression and consequent risk for AOD use in young Americans during the pandemic period, ³⁹ and the importance of social connection during the pandemic has been highlighted as pivotal in reducing harms associated with social distancing.^40,41 Difficulty in obtaining cannabis was also associated with using more frequently, which may be because these individuals are needing to obtain the drug more often or needing higher quantities overall. AOD treatment and harm-reduction services should take extra precautions to alleviate social isolation and loneliness via regular meaningful contact with clients with SUDs.

In contrast, we found that greater anxiety predicted using cannabis less frequently now compared with before the pandemic. Cannabis is well-known for eliciting anxiety after acute ⁴² and chronic use, ⁴³ and this finding may reflect using less frequently as a consequence of uncertainty around health during the pandemic period. Anxiety was also associated with using AOD earlier in the day more often, and using for boredom. Boredom may arise as a consequence of social distancing measures, and can be considered a stressor ⁴⁴ that is associated with negative affect and depression. ⁴⁵ Boredom is also a risk factor for relapse in SUDs.^46,47 Strategies to enhance other rewarding activities in the social isolation environment during the pandemic period may serve to attenuate the negative impact of boredom on AOD use and low mood, with special consideration for participants seeking support who demonstrate high levels of anxiety.

An unexpected finding was that participants with lower resilience were more likely to use methamphetamine either less or more frequently. Resilience has previously been positively associated with less COVID-19-related stress, anxiety or depression. ⁴⁸ Given that anxiety was also associated with less frequent cannabis use, it is possible that a subset of individuals with greater anxiety and reduced resilience experience a reduction in the frequency of use. Frequency and quantity of methamphetamine use may have also been affected by lower availability, as reported by 84% of people who reported using it in the current study. As the current study was cross-sectional, it is also possible that people who were experiencing changes in the frequency of cannabis or methamphetamine use may have experienced reduced resilience, as substance use is used as a coping mechanism throughout the pandemic.

Existing research has reported being anxious or depressed about COVID-19, loneliness, boredom, having more money, having more time and spending more time with others who inject drugs as the most common reasons given for increased use by people who inject substances. ¹⁷ The current study builds on this by demonstrating the role of mental health as associates of these, where depression, anxiety and resilience should be a primary consideration for assisting AOD treatment through the pandemic, given the increased rates of stress, financial pressures and social isolation. Mental health associates should be considered by treatment services throughout the pandemic via regular assessments with clients. Social-online or the continuation of mutual AOD support groups facilitated by telephones may also help reduce loneliness for this marginalised group.^49,50

Prior researchers have suggested two long-term trajectories of AOD use as a consequence of the pandemic. ³⁵ Declines in AOD use may be attributable to reduced social settings such as clubs and festivals.^16,34 Conversely, exacerbation of AOD use in dependent groups may be attributable to greater stress, financial pressure and uncertainty. ³⁵ The current study recruited people who had sought AOD treatment and reflects the latter group. These findings suggest that there are a subgroup of those, where important associates of changes in frequency of use include anxiety, depression and resilience, in addition to contextual factors (eg, loneliness and boredom). A large proportion of the sample did not experience changes to the frequency of use, and did not engage in more frequent AOD-related risk behaviours. This may indicate positive consequences of treatment during the pandemic, where greater efforts should be made to provide support and enrol new clients.

The study has several strengths and limitations. We recruited a sample of individuals who had sought AOD treatment in 2020, offering a unique opportunity to investigate the consequences of COVID-19 from this clinical perspective which can ultimately lead to enhanced care for SUDs. However, the small response rate (10%) may indicate selection biases that limit the generalisability of the findings, such as a self-selection bias and non-response bias. Further, a small and non-randomly stratified sample may limit the interpretability of the findings to a subsample of people seeking treatment through the pandemic. Nonetheless, the sample characteristics were representative of the full population of service client characteristics in terms of age, gender and mental health (normative data reported in SM2). Patterns of AOD use were lower than normative clients, yet the normative data reflects clients entering AOD treatment where it is expected that use would be higher. Alternatively, people with more severe use may have been less inclined to take part. Although some characteristics differed, the variation in relevant characteristics was sufficient to explore the aims of the study regarding the relation between mental health, contextual factors associated with AOD use and changes to AOD use during the pandemic. Our results may be underestimates given that individuals experiencing higher severity levels of psychological distress or substance use may be less likely to respond.

The current study highlighted the role of depression, anxiety and resilience in changes in the context and frequency of AOD use as a consequence of the pandemic, and emphasise the importance of different mental health concerns among people in treatment. People in treatment for AOD use problems are not a homogenous group, and these findings highlight the importance of understanding individual differences and contextual factors to reduce substance-related harms during the pandemic. Such research into the current pandemic climate is pivotal for informing treatment services internationally, where there are vast differences in social restrictions and the economic, health and social disruption caused by the COVID-19 pandemic is likely to be ongoing. This will likely have repercussions for the disease burden associated with substance use during a period where healthcare systems are already strained, but provide opportunities to enhance quality of life in AOD clients.