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,
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:
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
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.
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
Participants and setting
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
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
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
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 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
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).
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
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 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
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
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
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.
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.
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
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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