4.1 Principal findings
The current systematic review assessed the effectiveness of listening to the Quran on mental disorders and psychological well-being. The 11 included studies assessed its effectiveness on at least one of the four outcomes: anxiety, depression, stress, and overall mental health.
The current review showed a statistically and clinically important difference favoring listening to the Quran over no intervention on state anxiety, trait anxiety, and general anxiety in haemodialysis patients, pregnant women, patients waiting for a surgery, and athletes. Further, the evidence demonstrated that listening to the Quran is more effective than music and physical training in reducing state and trait anxiety and general anxiety in primiparous women and hemodialysis patients, respectively. However, all above-mentioned findings resulted from low or very low quality of evidence that had high risk of bias. In a systematic review conducted by Ghiasi & Keramat , the effect of listening to the Quran on anxiety were examined in 28 studies. Of these studies, only 4 were included in the current review. The remaining 24 studies were not included in the current review because either they were written in Persian language, or their full text was not available . Although Ghiasi and Keramat’s review did not meta-analyse results of the included studies, its conclusion was in line with our conclusion . To be more precise, Ghiasi and Keramat revealed that listening to the Quran is more effective than no intervention, music, and muscle relaxation in reducing anxiety in various settings . This finding supports our conclusion regarding the effect of listening to the Quran on anxiety.
Our review found a statistically and clinically important difference favoring listening to the Quran over no intervention on depression in haemodialysis patients and pregnant women. Listening to the Quran was also more effective than music and physical training in reducing depression in depressed women and hemodialysis patients, respectively. However, all aforementioned conclusion resulted from very low quality of evidence that had a high risk of bias. The effect of listening to the Quran on depression was not examined by any systematic review before the current review. However, a study excluded from the current review due to language (Persian) supported our findings . Specifically, Jaberi and colleagues revealed that listening to the Quran significantly reduced depression in depressed patients in comparison with no intervention . Similarly, an RCT found that spiritually integrated psychotherapy is more effective than no intervention in reducing depression in Muslims diagnosed with depressive disorder . Spiritually integrated psychotherapy consisted of daily lectures on Quranic verses, listening to the Quran, encouraging participants to recite the Quran . A cross-sectional survey excluded from the current review due to study design revealed that Iranian students who recite Quran have less depression than those how do not recite it .
In the current review, evidence from a single RCT showed a statistically significant difference favouring listening to the Quran over no intervention on mental health in medical staff. However, the risk of bias was rated as high in the RCT. Our findings are consistent with those of other studies excluded from the current review due to study design (quasi-experiment) , and language (Persian) . The former study demonstrated that listening to the Quran without its musical tone (Tartil) had a positive influence on mental health in Iranian personnel in a university . The latter study found that listening to the Quran significantly improved mental health in Iranian nursing students in comparison with no intervention .
Only one of the included studies revealed that listening to the Quran is more effective than no intervention in relieving stress in pregnant women. Yet, that evidence had a high risk of bias. Our findings are in line with what has been found by three studies excluded from the current review due to language (Persian) [44, 45] and study design (a cross-sectional survey) . The former two studies found a statistically significant difference favouring listening to the Quran over no intervention on stress in Iranian young people & adolescents  Iranian personnel in a university . The third study revealed that Iranian students who recite Quran have less stress than those how do not recite it .
The effect of listening to the Quran on mental disorders and psychological well-being may be attributed to the fact that listening to the Quran may increase religiosity and spirituality, which were found to have a positive effect on several mental disorders, such as depressive symptoms [46-50] and anxiety [51-54]. From a psychological perspective, the effect of the Quran can be explained by its ability to relax, reinforce and generate positive and pleasant thoughts, divert the mind from unpleasant irritations and ruminations, and restore harmony between the body and soul [31, 33]. The relaxing abilities of listening to the Quran may also result from its sound, meaning of its words, its miraculous combination of words within verses, and its rhythmic agreeable intonation [16, 33]. From a physiological perspective, Babamohamadi and colleagues believe that listening to the Quran may result in release of endorphins in the brain, which were found to affect mood and memory positively .
4.2 Strengths and limitations
The current study is the first systematic review that focused on effectiveness of listening to Quran on mental disorders and psychological well-being, meta-analysed results of studies, followed the PRISMA statement. Therefore, it is a more comprehensive, robust, transparent review than the previous reviews [11, 19, 20].
The review minimized the risk of publication bias through searching the most popular databases in healthcare field, searching the largest grey literature databases, conducting backward and forward reference list checking, using a comprehensive list of search terms, and not restricting the search to a certain year of publication, country of publication, and comparator.
To minimize the selection bias, two reviewers independently carried out the study selection, data extraction, risk of bias assessment, and quality of evidence evaluation. Interrater agreement was good for all processes, except for data extraction (which was very good). When possible, results of the included studies were meta-analyzed, and this improved the power of studies and increased the estimates of the likely size of effect of listening to the Quran on different outcomes.
The intervention of interest in this review was restricted to reciting or listening to the Quran, but not other Islamic spiritual interventions (e.g., supplications, prayers, mustahab praises). Further, this review focused on mental health-related outcomes and excluded outcomes related to physiological disorders or health. Accordingly, this review cannot comment on the effectiveness of other Islamic spiritual interventions and on the effect of listening to the Quran on physiological disorders or health.
The review was restricted to RCTs written in the English language, therefore, many studies were excluded because they were quasi experiments and/or written in the Persian language. This restriction was necessary because RCTs have higher internal validity than any other study design  and, given the resource available, it was not feasible to translate all non-English studies.
Most included studies recruited Iranian Muslims without mental disorders. This may limit the generalizability of our findings to non-Muslims with mental disorders in other countries. The overall risk of bias was rated as high in the all included studies, and quality of evidence for all meta-analyzed outcomes ranged from very low to low. Therefore, findings in this review must be interpreted with caution.
4.3 Practical and research implications
4.3.1 Practical implications
Although this study revealed that listening to the Quran is effective in improving anxiety, depression, stress, and mental health, it is difficult to draw definitive conclusions due to high risk of bias in the included studies, low quality of evidence, and lack of studies assessing each outcome. For this reason, health professionals and individuals should consider listening to the Quran as a complementary therapy to already available interventions for anxiety, depression, stress, and psychological well-being.
The effect of listening to Quran was smaller in studies that delivered the intervention only once [29, 37, 38] than those that delivered it more than once [30, 35]. Hence, people should consider listening to the Quran more than once to increase the effectiveness. Given that all chapters (Surah) and reciters used as an intervention in the included studies showed improvements in the measured outcomes, people should consider these chapters and reciters to improve anxiety, depression, stress, psychological well-being.
Most included studies used MP3 players delivering the intervention, and none of them used mobile applications (apps), which may be more accessible in the last decade. Thus, we encourage development and evaluation mobile apps that enable users to listen to the Quran and/ or Al-Ruqya Al-Shariya.
4.3.2 Research implications
The current review found relatively few studies assessing the effectiveness of listening to the Quran. Thus, researchers should conduct further studies to examine the effectiveness of listening to the Quran. They also should recruit non-Muslims with mental disorders from different countries given the lack such evidence.
While listening to the Quran was the intervention in all included studies, no study assessed the effect of reciting Quran by patients themselves. Further, no studies investigated the effectiveness of Al-Ruqya Al-Shariya. Accordingly, there is a need to assess the effectiveness of reciting the Quran by patients and Al-Ruqya Al-Shariya.
In most included studies, comparators were inactive intervention (i.e. no intervention). Future studies should assess the effectiveness of listening to the Quran in compassion with active interventions such as medications, music, yoga, and other spiritual or Islamic therapies.
In all included studies, outcomes were measured subjectively using self-administrated questionnaires. Therefore, participants’ answers might be biased to show a positive effect of listening to the Quran given that participants were Muslims in most studies and were aware of their treatment allocation. In addition to subjective outcome measures, researchers should objectively assess outcomes using, for example, electroencephalogram (EEG) to monitor changes in brainwaves.
Given that the overall risk of bias was judged as high in the included studies, we encourage researchers to follow recommended guidelines or tools (e.g., RoB 2  and CONSORT-EHEALTH ) when conducting and reporting their studies in order to minimize biases.