Regular Use of Medication for Musculoskeletal Pain
and Risk of Long-term Sickness Absence:
A Prospective Cohort Study Among
the General Working PopulationThis section is compiled by FRank M. Painter, D.C.
Send all comments or additions to: Frankp@chiro.org
FROM: European Journal of Pain 2017 (Feb); 21 (2): 366–373 ~ FULL TEXT
E. Sundstrup, M.D. Jakobsen, S.V. Thorsen, L.L. Andersen
National Research Centre for the Working Environment,
Copenhagen, Denmark.
Physical Activity and Human Performance group, SMI,
Department of Health Science and Technology,
Aalborg University, Denmark.
BACKGROUND: The aim was to determine the prospective association between use of pain medication - due to musculoskeletal pain in the low back, neck/shoulder and hand/wrist - and long-term sickness absence.
METHODS: Cox-regression analysis was performed to estimate the prospective association between regular use of pain medication and long-term sickness absence (LTSA; at least 6 consecutive weeks) among 9,544 employees from the general working population (Danish Work Environment Cohort Study 2010) and free from LTSA during 2009-2010. The fully adjusted model was controlled for age, gender, body mass index, smoking, leisure physical activity, job group, physical activity at work, psychosocial work environment, pain intensity, mental health and chronic disease.
RESULTS: In 2010, the proportion of regular pain medication users due to musculoskeletal disorders was 20.8%: 13.4% as over-the-counter (i.e. non-prescription) and 7.4% as doctor prescribed. In the fully adjusted model, regular use of over-the-counter [HR 1.44 (95% CI 1.13-1.83)] and doctor prescribed (HR 2.18 (95% CI 1.67-2.86)) pain medication were prospectively associated with LTSA.
CONCLUSIONS: Regular use of pain medication due to musculoskeletal pain is prospectively associated with long-term sickness absence (LTSA) even when adjusted for pain intensity. This study suggests that use of pain medication can be an important factor to be aware of in the prevention of sickness absence. Thus, regular use of pain medication - and not solely the intensity of pain - can be an early indicator that musculoskeletal pain can lead to serious consequences such as long-term sickness absence.
SIGNIFICANCE: Use of medication due to musculoskeletal pain is prospectively associated with long-term sickness absence even when adjusted for pain intensity. Use of pain medication can be a red flag to be aware of in the prevention of sickness absence.
From the FULL TEXT Article:
Introduction
Sickness absence represents a major global health and economic challenge emphasized by the 190 million workdays lost per year in the public sector in the United Kingdom alone (Chartered British Institute, 2010). Thus, sickness absence is a topic of political concern in the European Union (EU) and strategies to reduce absence from work are highly prioritized.
Musculoskeletal disorders represent the most common occupational disease in the EU, (European Agency for Safety and Health at Work, 2010) with consequences ranging from poor health and work disability to sickness absence and loss of productivity (Hansson and Hansson, 2005; Andersen et al., 2011, 2012a, 2012b). Specifically, low back pain, neck/ shoulder pain and hand/wrist pain are known risk factors for sickness absence (Morken et al., 2003; Bergstrom et al., 2007; Nyman et al., 2007; Andersen et al., 2011). Initiatives to prevent and rehabilitate musculoskeletal pain in these regions are therefore warranted in the global pursuit of reducing sickness absence. Perceived pain intensity is often used for assessing seriousness of musculoskeletal disorders. However, due to the subjective nature of perceived pain it may be relevant to look also for objective or factual predictive factors for sickness absence, e.g. use of pain medication.
The use of pain medication is common in the general population as well as in the working population (Porteous et al., 2005; Mortensen et al., 2007; Rossignol et al., 2009; Dale et al., 2015; Samuelsen et al., 2015). The use of pain medication among employees may express difficulties in managing to work with pain, and can reflect a strategy to cope with symptoms that are partially work-related (Kristensen, 1991; Ajslev et al., 2013). The growing accessibility of non-prescription analgesics in many countries enables employees to self-manage health conditions, such as musculoskeletal pain, without medical consultation (Cusack et al., 2013). However, unsafe use of pain medication can cause adverse events and knowledge on correct use and alternative treatment options seems prevalent. Hence, there is a growing need to investigate the consequences of pain medication due to musculoskeletal pain on long-term sickness absence in the general working population, to thereby design more efficient and effective prevention strategies and societal campaigns.
Through merging of data from the Danish Work Environment Cohort Study (DWECS) and the national register of social transfer payment (DREAM), this study estimates the prospective association between regular use of medication – due to musculoskeletal pain in the low back, neck/shoulder and hand/wrist – and long-term sickness absence in the general working population.
Methods
Study design
This study estimates the prospective association between use of pain medication and long-term sickness absence (LTSA) through merging of data from the Danish Work Environment Cohort Study (DWECS) and the national register of social transfer payment (DREAM).
Participants and setting
Table 1 Data on health and work environment used in the study population were obtained from the 2010 round of the Danish Work Environment Cohort Study (DWECS) (Burr et al., 2003). This study consists of a survey assessing work environment and health in the general working population of Denmark and has been repeated every fifth year since 1990. The questions on pain and use of pain medication are specified below. DWECS consists of approximately 21,000 individuals aged 18–59 years from the general working population in Denmark, randomly sampled from the Central Register of Denmark. A total of 10,605 workers (~53%) replied to the 2010 questionnaire survey (Nabe- Nielsen et al., 2015). For the analyses in the present study, we included only currently employed wage earners that were not on longterm sick leave during 2009 and 2010 (N = 9,544). Because not all participants filled in all questions, the exact number for each analysis varies. Table 1 shows the baseline characteristics of the study population.
Ethics approval
The study was notified and registered by the Danish Data Protection Agency (journal number: 2007-54- 0059). According to Danish law, questionnaire-based and register-based studies do not need approval by ethical and scientific committees, nor do they need informed consent. (The Danish Data Protection Agency, 2008; Committee System on Biomedical Research Ethics, 2011). All data were de-identified and analysed anonymously.
Predictor variable
Use of pain medication due to musculoskeletal disorders was assessed by the question ‘have you during the last 12 months used pain medication on a regular basis for periods of at least 14 days due to the aforementioned pain’ [i.e. neck-shoulder, low back and hand-wrist pain] (Sundstrup et al., 2016). The response options were ‘Yes, prescription medication’, ‘Yes, over-the-counter medication’ and ‘No’. For subsequent analyses, the two-first options were collapsed to ‘Yes’.
Outcome variable
Information on sickness absence was derived from a Danish register of social transfer payments (DREAM), (Lund et al., 2008) and linked to the Danish Work Environment and Cohort Study via the unique personal identification number which is given to all Danish citizens at birth. The Danish Register for Evaluation of Marginalization (DREAM) contains information on all transfer payments (including sickness absence compensation, employment, early retirement, government education, unemployment benefits, etc.) and other basic personal data for all Danish residents on a weekly basis (Burr et al., 2011). DREAM has high reliability, since all transfer payments are systematically recorded in DREAM and employers have a financial incentive to report sick leave as they can apply for compensation of employee sickness absence costs after 30 days of sick leave. Questionnaire data from DWECS were prospectively linked to DREAM by the unique social security number, given to all Danes at birth. In the present study, we defined long-term sickness absence as having registered at least 6 consecutive weeks in the weekly based DREAM register in the 2-year follow-up period. The DWECS cohort was followed up for 2 years after the baseline year (2010) in the DREAM register, i.e. data on LTSA were extracted from 2010 to 2012.
Control variables
Control variables included age, body mass index (BMI), gender, smoking status (‘No, never’, ‘Ex-smoker’ and ‘Yes’), job group, physical activity at work (described below), psychosocial work environment (described below), physical activity during leisure (described below), musculoskeletal pain (described below), mental health (from the SF-36 questionnaire (Bjorner et al., 1998)), and chronic diseases (described below).
Physical activity during leisure was assessed by the question ‘How much time have you spent on each of the following leisure time activities during the last year (including commuting to and from work)?’ (1) ‘Walking, biking or other low-intensity exercise, where you do not get short of breath and do not begin to sweat (e.g. Sunday walks or low-intensity gardening)?’; (2) ‘Exercise training, heavy gardening, or higher intensity walking/biking, where you sweat and get short of breath?’ and (3) ‘Strenuous exercise training or competitive sports?’ The response options for each sub-question were: ‘>4 h/ week’, ‘2–4 h/week’, ‘<2 h/week’ or ‘do not perform this activity’. We defined low, moderate and high physical activity the following way: low physical activity, performing <4 h of low-intensity physical activity per week, and not performing moderateintensity and high-intensity activities at all; moderate physical activity, performing more than 4 h of low-intensity physical activity per week or moderate activity for <4 h/week or high-intensity activity for <4 h/week; high physical activity, performing moderate or high activity for >4 h/week, or a combination of moderate and high activity for 2–4 h/week (Andersen et al., 2016).
Psychosocial work environment was assessed by four dimensions, each including a number of questions, from the Copenhagen Psychosocial Questionnaire (COPSOQ) (Pejtersen et al., 2010). The four psychosocial work environment dimensions included influence at work, emotional demands, support from colleagues and support from superiors.
Musculoskeletal pain intensity was assessed as average pain during the last 3 months on a scale of 0–9, where 0 is no pain and 9 is worst pain, for the neck-shoulder, low back and hand-wrist, respectively. The pain question was phrased as ‘trouble (pain or discomfort)’ (Andersen et al. 2010).
Chronic disease was assessed by the following question, ‘Have you ever been informed by a physician that you have or have had one or more of the following conditions?’ with the response options being ‘Yes’ and ‘No, never’ to the following diseases: Depression, asthma, diabetes (all types), cardiovascular disease, cancer, impaired hearing, eczema, back disease or other diseases (Calatayud et al., 2015).
Statistics
Cox proportional hazard model was used for modelling the probability of long-term sickness absence (≥6 weeks) during the 2-year follow-up period, with regular use of over-the-counter and prescribed pain medication as explanatory variables. Model 1 was adjusted for age and gender. Model 2 was the same as model 1, but additionally included job group, physical activity at work and psychosocial work environment (influence at work, emotional demands, support from colleagues and support from leader/superiors). Model 3 was the same as model 2, but additionally included lifestyle (smoking, leisure physical activity, BMI). Model 4 was the same as model 3, but additionally included pain intensity in the neck-shoulder, low back and hand/wrist. Model 5 was the same as model 4, but additionally included mental health. Model 6 was the same as model 5, but additionally included chronic disease. The data on LTSA correspond to survival times, and the cohort was followed up for 2 years after the baseline year, i.e. 2011 and 2012. However, respondents were censored in case of retirement, disability pension, immigration or death. When individuals had an onset of LTSA within the follow-up period, the survival times were non-censored and referred to as event times. Results are reported as hazard ratios (HRs) with 95% CIs and the estimation method was maximum likelihood.
Results
Table 1 shows the descriptive statistics for the main study variables. Of the 9,544 participants, 1,252 (13.4%) and 694 (7.4%), respectively, used overthe- counter, and doctor prescribed pain medication due to musculoskeletal pain in the low back, neck/ shoulder and hand/wrist.
Table 2 Table 2 shows prospective associations between use of medication (over-the-counter or prescribed by a doctor) for musculoskeletal pain and risk of longterm sickness absence among the general working population. In model 1, adjusting for age and gender, regular use of over-the-counter and doctor prescribed pain medication increased the risk for long-term sickness absence by 110% and 191%, respectively. Similar results were found in model 2, with additional adjustment for job group, physical activity at work and psychosocial work environment. In model 3, with additional adjustment for smoking, leisure time physical activity and BMI, the risk estimates decreased but remained highly significant, and regular use of over-the-counter and doctor prescribed pain medication increased the risk for longterm sickness absence by 81% and 176%, respectively. In model 4, with additional adjustment for pain intensity in the neck-shoulder, low back and hand/wrist, the risk estimates decreased further but were still significant. In this model, regular use of over-the-counter and doctor prescribed pain medication increased the risk for long-term sickness absence by 48% and 125%, respectively. Similar results were found in model 5, with additional adjustment for mental health. Additional adjustment for chronic diseases in the final model (model 6), did not change the risk estimates, and regular use of over-the-counter and doctor prescribed pain medication increased the risk for long-term sickness absence by 44% and 118%, respectively. All six models showed approximately a doubling of the risk estimates for long-term sickness absence from regular use of over-the-counter compared to doctor prescribed medication due to musculoskeletal pain.
Discussion
In this study, we prospectively followed 9,544 employees from the general working population with varying use of pain medication due to musculoskeletal pain in the national register of social transfer payment (DREAM). The results show an increased risk for long-term sickness absence from regular use of medication due to musculoskeletal pain in the low back, neck/shoulder and hand/wrist. Importantly, even when adjusting for intensity of musculoskeletal pain, medication use due to musculoskeletal pain is still a strong predictor for LTSA. Use of pain medication – and not solely the intensity of pain – can be an important factor to be aware of in the prevention of sickness absence. Thus, regular use of pain medication can be an early indicator that the specific pain can lead to serious consequences such as sickness absence.
Before we begin the discussion of the study results, we will first address some strengths and limitations. Due to Danish law, the DREAM register holds no information on the causes of sickness absence, which could limit any causal inference between regular use of pain medication due to musculoskeletal disorders and long-term sickness absence caused by a specific disease or illness. This study is therefore only able to give risk estimates for all cause LTSA due to regular use of pain medication for musculoskeletal pain. All cause sickness absence could therefore reflect a behavioural trait or propensity to take sick leave rather than more specific reasons (e.g. regular use of pain medication).
The results of the present study could have been influenced by selection bias due to a considerable number of invitees declining to participate in the DWECS questionnaire survey (53% response rate). Previous research has shown that unhealthy persons attend to a lesser degree than healthy individuals in population-based surveys (Søgaard et al., 2004). Altogether, this could lower the risk estimates for the remaining workers regularly using pain medication, and our estimates may therefore be conservative. Use of pain medication was self-reported and could therefore have been influenced by recall or reporting bias. However, using medical registers for pain medication consumption could also be biased since discrepancies seem to exist between what is sold as over-the-counter or prescribed by a doctor and the actual consumption of pain medication. Importantly, no medical register holds information on non-prescription pain medication, emphasizing the necessity for self-reports when investigating self-management of health conditions such as musculoskeletal pain.
The Cox models in the present study were constructed by a step-wise adding of potential confounders, which could have influenced the procedure, as some could have been on the causal pathway (i.e. over adjustment). In the present study, we choose first to adjust for workrelated factors, then lifestyle, and finally healthrelated factors. A strength of the study is the use of information on sickness absence from the DREAM register, which has a high validity since employers have an economic incentive to report sickness absence as employers can apply for compensation of sickness absence costs after 30 days of sickness absence. This inherently eliminates any recall or reporting bias. Another strength of the study is that the analysis included only currently employed wage earners that were not on LTSA at baseline and the previous 2 years. This inherently eliminates any bias that would arise specifically if those on LTSA reported their use differently from those who were not on sick leave. Finally, the generalizability of the study is increased by the use of a representative sample of the general working population in Denmark.
Regular use of doctor prescribed pain medication due to musculoskeletal pain was a stronger predictor for long-term sickness absence compared with use of over-the-counter pain medication, and was in the fully adjusted model associated with an 118% increased risk among the general working population. Antonov et al. showed, in a cross-sectional study design among approximately 12,000 Swedes that poor self-rated health and high use of health care services were associated with increased use of prescription but not to non-prescription (i.e. overthe- counter) pain medication (Antonov and Isacson, 1996). Hence, it could be speculated that those seeking professional advice from a doctor are more affected by their pain compared to the fraction of workers who turns to self-management of pain symptoms. This could possibly explain the increased risk of long-term sickness absence due to regular use of doctor prescribed compared with over-the-counter pain medication observed in the present study. In addition, it cannot be excluded that visiting a doctor could reflect a gateway to taking long-term sick leave, as employees might need a medical certificate, which may require a medical prescription to legitimize the event. For example, in Denmark it is the employers’ right to ask the employee for proof of sick leave by a medical certificate. This scenario could also have influenced the observed differences in the risk estimates between regular users of nonprescription and prescription pain medication.
Blue-collar workers show higher rate of chronic and continuous utilization of non-selective nonsteroid anti-inflammatory drugs (NSAIDs) than white-collar workers (Rossignol et al., 2009), and blue-collar workers also demonstrate increased risk of sickness absence due to musculoskeletal disorders compared with white-collar workers (Morken et al., 2003). Also workers with high physical demands are at increased risk for long-term sickness absence (Lund et al., 2006) and previous research has presented an association between perceived psychosocial work environment and sickness absence (Voss et al., 2001; Lund et al., 2005; Clausen et al., 2012). Specifically, workers with higher levels of physical activity at work have shown to be more likely to use pain medication on a regular basis for musculoskeletal pain compared with workers with seated work (Sundstrup et al., 2016). In the present study, we therefore controlled for job group, physical work environment (activity at work) and psychosocial work environment (influence at work, emotional demands, support from colleagues and support from leader/superiors). Adjusting for these factors in model 2 reduced the risk estimate only slightly for use of over-the-counter pain medication, whereas the risk estimate for use of doctor prescribed medication was unaffected. Hence, there seems to be no indication that a good working environment can reduce the risk of long-term sickness absence, at least among the workers using doctor prescribed pain medication.
Controlling for lifestyle factors such as smoking, leisure physical activity and BMI, reduced the risk estimates by approximately 10% (model 3), whereas adjusting for mental health (model 5) or chronic diseases (model 6) did not change the risk estimates further. The latter could be explained by the fact that pain intensity was already controlled for in the model.
Andersen et al., demonstrated that pain intensity ≥4 on a 0–10 scale in the low back, neck/shoulders and hand/wrist are risk factors for future sickness absence (Andersen et al., 2011). Furthermore, pain intensity has been proven a strong predictor of overthe-counter analgesics use in individuals with chronic as well as non-chronic pain (Dale et al., 2015). As expected, controlling for pain intensity in the low back, neck/shoulder and hand/wrist reduced the risk estimates in the present study considerably (model 4).
However, over-the-counter and doctor-prescribed pain medication remained significant and increased the risk for long-term sickness absence by 44% and 118%, respectively, even in the full-adjusted model (model 6). Use of pain medication – and not solely intensity of pain – can be an important factor to be aware of in the prevention of sickness absence. Specifically, the binary nature of the question (either you use pain medication or you don’t) makes it a factual, feasible and easily recognizable predictor (that also captures the consequence of pain – difficulties in managing pain) that calls for action in contrast to the subjective nature of pain intensity that previously has been a source of criticism (Kamper, 2012; Ballantyne and Sullivan, 2015).
Additionally, Ballantyne et al. recently argued that numerical ratings of pain intensity could be the wrong metric for the treatment of chronic pain and that ‘multiple measures of the complex causes and consequences of pain are needed to elucidate a person’s pain and inform multimodal treatment’ (Ballantyne and Sullivan, 2015). Regular use of pain medication among employees may express difficulties in managing to work with pain, and could reflect a strategy to cope with symptoms that are partially work-related (Kristensen, 1991; Ajslev et al., 2013). It is well established that pain is a complex biopsychosocial phenomenon and many factors can therefore influence regular use of medication due to musculoskeletal pain (Gatchel et al., 2007). Thus, the high-risk estimates for LTSA, found in the present study, could also be mediated by factors such as pain behaviour and beliefs. Overall taken, regular use of pain medication can be an early indicator that pain can lead to serious consequences – such as longterm sickness absence – and that relevant treatment options and preventative initiatives are needed.
Conclusions
Use of pain medication due to musculoskeletal pain increases the risk for long-term sickness absence among 9,544 employees from the general working population in Denmark. Importantly, even when adjusting for musculoskeletal pain, medication use was still a strong predictor for long-term sickness absence, suggesting that regular use of pain medication can be an important factor to be aware of in the prevention of sickness absence.
Acknowledgements
The authors are grateful to colleagues Elsa Bach and Ebbe Villadsen at NRCWE for valuable discussions and assistance with access to data from the Danish Work Environment Cohort Study.
Author contributions
LLA and ES designed the study and LLA performed the analyses. SVT and MDJ provided feedback on the study design, analyses and interpretation of data. ES drafted the manuscript, and all the authors provided critical feedback and approved the final version.
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