Introduction
Various studies have reported a prevalence of chronic pain in almost 15% of the general population, while the World Health Organization (WHO) estimate is 20% [1].
Although there is no definitive cure for chronic pain, solving some of its problems, including anxiety, depression, and reduced quality of life can lessen the overall effect of pain on the patient’s life and increase their quality of life. Persistent pain causes a wide range of problems, including mood disorders, such as depression, persistent rumination about pain, personal limitations in social and occupational activities, increased arbitrary use of medication, frequent visits to health departments, and the spread of the patient’s condition to other areas of their life [4].
Despite the moderate effectiveness of some biological and psychological therapies for pain, chronic pain is still a stressful and debilitating illness for many people, and there has never been a complete and successful cure for it. For this reason, therapists attempt to reduce the patient’s pain by combining different treatment techniques or expanding the existing treatments. Psychological therapies for chronic pain include behavioral therapy, biofeedback and relaxation, cognitive-behavioral therapy, group therapy, and acceptance and commitment therapy; meanwhile, mindfulness-based stress reduction (MBSR) therapy is a novel method.
Methods
This was a quasi-experimental study with a pre-test, post-test, and control group. The inclusion criteria were the presence of at least one symptom of pain without any specific medical reason for at least 6 months, being in the age range of 18 to 60 years, and having the education level of at least middle school (8th grade). The exclusion criteria were substance abuse and severe psychiatric problems, such as psychosis.
The statistical population of this study consisted of all patients with chronic pain referred to Imam Reza Hospital of AJA University of Medical Sciences. Among these patients, 50 cases who met the inclusion criteria and were willing to participate in the study were selected through convenience sampling and randomly assigned to the experimental and control groups. 
Based on the announcement and invitation that was installed in Imam Reza Hospital for participating in this research, the applicants called and expressed their desire to enroll. After the initial screening, the patients were informed about the purpose of the study, and an informed consent letter was obtained from each participant. Following the consent, the therapist conducted a structured clinical interview for the diagnostic and statistical manual of mental disorders, 5th edition (DSM-5) (SCID-5). If the participants complied with the inclusion and exclusion criteria, they entered the study as a research sample. Accordingly, 50 patients were selected. After selecting the patients and randomly assigning them to experimental and control groups, the research questionnaires were given to the patients before and after the treatment. The experimental group received MBSR therapy (8 weekly sessions) and the control group did not receive any intervention. In this study, the following questionnaires were used to measure the clinical and demographic characteristics: 
• Structured clinical interview for DSM-5 (SCID-5);
• World Health Organization quality of life–BREF (WHOQOL-BREF);
• Beck anxiety inventory (BAI);
• Beck depression inventory-2nd edition (BDI-II);
• Numeric pain rating scale (NPRS).
Results
The participants in this study included 27 women (57.4%) and 20 men (42.6%) who were in the age range of 28 to 60 years (with an average of 48.68 years and a Standard Deviation (SD) of 8.88 years). Among all the participants, 23 (48.9%) were in the experimental group and 24 (51.1%) were in the control group. Table 1 presents the frequency, the frequency percentage, and the results of the Chi-square test for the gender variable in the two groups.


In Table 2, the mean, standard deviation and results of the independent Student's T test for the variables of pain intensity, anxiety, depression and quality of life before the intervention are presented separately for two groups.


Regarding the pain intensity variable, according to the value of the covariance analysis test (37.260) and its significance level (P˂0.0001) as seen in Table 3, a significant difference exists between the two groups and it is determined based on the descriptive indicators that the mean post-test scores of the pain intensity in the experimental group (4.30) are lower than the control group (6.58).


In addition, the effect size of 0.459 in Table 3 shows that approximately 46% of the variance of the pain intensity variable is explained by the grouping variable (i.e., type of treatment). The power of the test (1.00) also indicates the high power of the statistical test in detecting the differences between the two groups.
Regarding the quality-of-life variable, according to the value of the covariance analysis test (28.889) and its significance level (P˂0.0001) as seen in Table 3, a significant difference exists between the two groups and it is determined based on the descriptive indicators that the mean scores of the post-test quality of life in the experimental group (53.57) are higher than the control group (48.71). In addition, the effect size of 0.396 in Table 3 shows that approximately 40% of the variance of the quality-of-life variable is explained by the grouping variable (i.e., type of treatment). The power of the test (1.00) also indicates the high power of the statistical test in detecting the differences between the two groups.
In the anxiety variable, according to the value of the covariance analysis test (21.388) and its significance level (P˂0.0001) as seen in Table 3, a significant difference exists between the two groups, and based on the descriptive indicators it is determined that the average posttest scores of anxiety in the experimental group (20.96) are lower than the control group (26.04). In addition, the effect size of 0.327 in Table 3 shows that approximately 33% of the variance of the anxiety variable is explained by the grouping variable (i.e., type of treatment). The test power of 0.995 also indicates the high power of the statistical test in detecting the differences between the two groups.
In the variable of depression, according to the value of the covariance analysis test (23.579) and its significance level (P˂0.0001) as seen in Table 3, a significant difference exists between the two groups and it is determined based on the descriptive indicators that the mean posttest scores of depression in the experimental group (13.83) are lower than the control group (19.46). In addition, the effect size of 0.349 in Table 3 shows that approximately 35% of the variance of the depression variable is explained by the grouping variable (i.e., type of treatment). The test power of 0.997 also indicates the high power of the statistical test in detecting the differences between the two groups.
Discussion
Chronic pain goes far beyond a physical symptom. Psychological consequences of having chronic pain include fear, anxiety, hopelessness and depression, and reduced quality of life. Some researchers believe that a depressed mood reduces the pain tolerance threshold [1]. Nash et al. explained the interaction of cognitive, emotional, and pain factors in predicting the inability of patients with headaches, among which, the role of pain-related anxiety is more prominent in the disability of patients with headaches [2]. Accordingly, on the one hand, pain leads to negative emotions, and on the other hand, negative emotion causes pain to persist. This causal effect is one of the most important factors in experiencing pain [3]. Mindfulness techniques are effective in increasing muscle relaxation and reducing anxiety and stress. The main mechanism of mindfulness seems to be the self-control of attention because repeated focusing of attention on a neutral stimulus, such as breathing creates an appropriate attention environment [1].

Ethical Considerations
Compliance with ethical guidelines
The present research in terms of compliance with ethical guidelines has been approved in the ethics committee of AJA University of Medical Sciences with this code of ethics: IR.AJAUMS.REC.1397.052.

Funding
This research had been financially supported by the National Elite Foundation and AJA University of Medical Sciences.

Authors contributions
All authors contributed equally to the preparation of this article.

Conflicts of interest
The authors declared no conflict of interest.


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