Pain is a growing public health priority. Demographic changes, rising comorbidity rates, the difficulty of making accurate diagnoses, and poor access to treatment are some of the challenges faced by healthcare systems in the management of chronic pain. Among all the treatment options, opioids have played an increasingly important role in the control of moderate to severe pain (Goldberg and McGee, 2011; Majors-Foley, 2016; Andrews et al., 2018).

The main indication of strong opioids is for acute pain of moderate to severe intensity, especially for oncological or postoperative pain (World Health Organisation, 1996). Due to their known side effects, they must be monitored under the strict control of a healthcare professional (Labianca et al., 2012). Recent data have shown an increase in opioid consumption for the treatment of chronic non-cancer pain (CNCP), including osteoarthritis of the extremities and spondylarthritis as the most common pathologies (Agència de Qualitat i Avaluació Sanitàries de Catalunya, 2018). Although opioids are useful for alleviating pain, their use and side effects have not been thoroughly studied in the case of many pathologies that cause CNCP, and little evidence has been reported on the risk-benefit balance in these cases. Even so, many clinical practice guidelines maintain that these drugs should be second-line drugs (that is, administered under strict monitoring by a healthcare professional) due to their negative effects on cognition and their addictive potential (Kahan et al., 2011; Labianca et al., 2012; Manchikanti et al., 2012; Scottish Intercollegiate Guidelines Network (SIGN), 2013; Agència de Qualitat i Avaluació Sanitàries de Catalunya, 2018).

Given the generalised increase in consumption, various indicators of misuse (including addiction rates or overdose-related deaths) have been described in different contexts, and have revealed similar trends (Kalkman et al., 2019; Schieber et al., 2019). This trend has been observed on a large scale in the United States, the country with the most opioid users in the world: between 1999 and 2007, sales of opioid drugs increased by 149% (Manchikanti et al., 2013), and there were 399,230 overdose deaths (Scholl et al., 2018). As a result, in 2017 the US declared this issue a public health emergency and opioid abuse is still considered an epidemic today (US Department of Health and Human Services, n.d.).

Though less pronounced than in the United States, a similar trend has been observed in Europe in the last few years (Bosetti et al., 2019; European Monitoring Centre for Drugs and Drug Addiction, 2020). Increases in opioid use have also been recorded in Spain, where consumption rose by 88% between 2010 and 2018, with a notable increase in the use of strong opioids (Agencia Española de Medicamentos y Productos Sanitarios, n.d.). The same trend is observed in Catalonia, where total use of opioids between 2012 and 2016 increased by 45% (Pando et al., 2017). Among strong opioids, the rise in fentanyl use was particularly marked.

An issue that has not been widely addressed so far is the presence of inequalities in opioid use. The analysis of the social determinants of opioid use has been paid far less attention than clinical characteristics and age as the main determinants of its increase, despite the reports of ethnic, socioeconomic, gender, and geographical variations (Morden et al., 2014; Song, 2017; Santoro and Santoro, 2018; Friedman et al., 2019; Schieber et al., 2019; Park and Wu, 2020).

The aim of the present study is to analyse the geographical, socioeconomic, and gender determinants of the use of opioids for CNCP in Catalonia.

We assessed the results of this study from two perspectives: first, focusing on the socioeconomic dimension, and second, focusing on gender.

This study reveals a strong geographical and socioeconomic gradient in opioid use in Catalonia. The results showed that the DDD/1,000 inhabitants per day was 10 times higher in the HA with the highest use than in the one with the lowest. The most striking case was the use of weak opioids in women, where the lowest rate (2.34 DDD/1,000 inhabitants per day) was recorded in the Barcelona 5-C HA (an affluent district of the city), and the highest rate (22.18) was recorded in the Badalona 5 HA (a deprived area). In addition, the descriptive maps show that in metropolitan areas the inequalities were more evident, both in terms of socioeconomic status and in opioid use. This finding was especially notable in the metropolitan area of Barcelona, but it was also observed in the cities of Girona, Lleida, and Tarragona, three of the region’s major urban areas.

These socioeconomic differences in opioid use could be explained by two factors. The first possible explanation concerns the differences between the individual socioeconomic characteristics of the inhabitants of the different HAs. Previous literature reports have highlighted the wide-ranging impact of socioeconomic conditions on the population’s health (García-Altés et al., 2018; Kivimäki et al., 2020). In the specific case of CNCP, the risk of suffering this type of pain is higher in unfavourable socioeconomic conditions (Riskowski, 2014; Grol-Prokopczyk, 2017; Jonsdottir et al., 2019), a circumstance that may increase opioid prescription. The second explanation relates to the differences in the characteristics of each HA. Factors such as the concentration of poverty and social segregation, the physical structure of the environment (e.g., adaptability for people with disabilities, accessibility to public transport, etc.) or the network of public facilities may have a decisive influence on the prevalence of morbidities and pain (Marí-Dell’Olmo et al., 2016; Rocha et al., 2017).

Other important issues for debate are environmental or individual differences such as the access to supplementary treatments (such as additional physiotherapy or exercise programmes not covered by public health services) and the access to pain treatment specialists in different HAs. These treatments may be less accessible to some people, either for economic reasons, lack of time, or because they cannot prioritize their own care. Differences between HAs may also be driven by the availability of multidisciplinary services in pain management. In this context, the results of the study suggest that opioid analgesics have become a fast and economical way for the Catalan health system to treat CNCP. In addition, opioid management differs according to the approaches of the medical teams and the guidelines in place in each HA, which may explain some of the differences in use patterns. Previous studies in the United Kingdom, which like Catalonia has a universal healthcare system, have identified similar risk factors in opioid prescribing, including the lack of time spent on dedicated patient care and socioeconomic deprivation (Curtis et al., 2019). In contrast, one study in Mexico showed that areas with a higher socio-economic status had higher rates of opioid dispensing after adjusting for palliative care need. Factors such as the coexistence of different health systems and differences in access both to medicines and to health services may be the main reasons for these disparities, with the most vulnerable areas being the ones with the lowest provision of these services (Goodman-Meza et al., 2021). The studies just mentioned and the present one all highlight the importance of the health system as a guarantor of equitable access to pain treatment and the use of opioid drugs. Another noteworthy finding is that the socioeconomic gradient is more pronounced in intermediate ages, especially in the 45–64 years range, where in many cases the increase in use between the most and the least favoured areas exceeds 100%.

This study found a more pronounced socioeconomic gradient in the prescription of weak than of strong opioids. This may be due to the fact that the prescription of strong opioids tends to be more protocolised and controlled, which may reduce the differences in consumption between socioeconomic groups. Likewise, in the prescription of opioids for a cancer diagnosis, socioeconomic status has a much lower impact than in the case of a diagnosis of non-oncological pain. There are two possible explanations for this. First, cancer is a particularly sensitive issue for the healthcare system and its treatment follows clearly established guidelines, with the result that the influence of socioeconomic factors is probably less important than in other situations. Second, the data on drug prescription used in this study corresponded to drugs dispensed at a community pharmacy office; it was not possible to obtain data on consumption in the hospital or long-term care setting. Despite the fact that socioeconomic status has a lesser impact on the prescription of opioids for oncological pain, our results indicated some notably high values in the most disadvantaged HAs, especially in the prescription of strong opioids. On this point, previous research has pointed out that an individual’s socioeconomic status affects their risk of developing cancer (Mihor et al., 2020).

From a gender perspective, the results show huge differences in the consumption of opioids between men and women. The issue of gender inequalities in health has been extensively studied; women have been found to present worse indicators of perceived health and higher prevalence of certain pathologies (Mauvais-Jarvis et al., 2020) and pain (Fillingim et al., 2009; Andrews et al., 2018; Sylwander et al., 2020). Moreover, women tend to make greater use of health services (García-Altés et al., 2018; Mihor et al., 2020). Regarding opioid consumption, areas with lower Gender Development Index scores have the worst results in analgesic management indicators. Women are prescribed more opioids than men but make fewer visits to specialist doctors (Chilet-Rosell et al., 2013). Differences in attitudes to opioid management between healthcare units and professionals have also been reported (Green et al., 2003; Perelló Bratescu et al., 2020). What remains unclear is the relative contribution of biological sex to pain mechanisms and its distribution and the contribution of gender (Fillingim et al., 2009). A gender perspective approach is needed to accumulate more evidence and clarify its social influence to chronic pain and its management (Serdarevic et al., 2017; Mauvais-Jarvis et al., 2020).

In interpreting the results, the greater proportion of women in the population should be borne in mind. This difference becomes wider at more advanced ages, as women in the over-75 age group outnumber men by 50%. However, data on individual opioid users showed that, in the same population, there were more female than male users in absolute and relative terms for both strong and weak opioids, thus maintaining the gender differences.

The results of the study show that the socioeconomic gradient in opioid use is much more pronounced in women than in men. Gender should be established as a key social factor in opioid consumption for two reasons: the gender bias in the health system and the poorer health status of women, partly due to a higher manifestation of pain symptoms and partly due to their less favourable socioeconomic status. These dimensions are intersectional and explain, among other things, the large differences observed between men and women (Serdarevic et al., 2017; Mauvais-Jarvis et al., 2020).

This study has some limitations. First of all, as it is an ecological study, it is impossible to determine the effect that the various factors have on the use of opioid analgesics at the individual level. Also, within an HA with an assigned ISC, there might be individuals with very different characteristics. Second, the 1 year time period used in the study does not allow us to determine trends in opioid use. Third, cancer prevalence and the patients-per-physician ratio in each HA were not taken into account. Finally, the data corresponds to the drugs dispensed by the public health system in pharmacies. Therefore, we do not have data on private consumption (i.e., through private insurance or other sources), or data on hospitals or long-term care prescriptions.

The data on opioid prescribing should also be interpreted with care. A high opioid use does not necessarily reflect inadequate pain control management; it may represent a satisfactory response of the health system to the requirements of the population. The aim of this study is to highlight the gap existing between use rates due to different socioeconomic and sociodemographic factors, and is certainly not to criticise legitimate medical uses.

Despite the limitations, this study presents a number of strengths. We used administrative medical databases that include records of all public healthcare services and prescriptions dispensed for the whole Catalan population. These sources provided valuable information and are increasingly used as a robust tool in public health research.

To conclude, our results shed light on the social differences existing between geographical areas in the management of opioid use in CNCP. Our findings calls for stronger action to promote best practices in prescribing for chronic pain and to reduce socioeconomic and gender variation between geographical areas. This study also provides a model for routine monitoring of opioid prescribing in order to promote interventions able to reduce high-dose consumption in identified HAs.