Intestinal motility and transit time are the primary determinants of gut microbiota composition (Zádori et al., 2023). Constipation induced by opioid therapy and compromised intestinal motility exerts a profound influence on the composition of the gut microbiome. In a mouse model of long-term morphine treatment, dysbiosis of the gut microbiota was observed (Table 2). This dysbiosis comprised an increase in the abundance of Staphylococcus, Enterococcus, and Proteus, alongside a decrease in the abundance of Bacteroidales, Clostridiales, and Lactobacillales (Akbarali and Dewey, 2017; Kim et al., 2021). Furthermore, alterations in gut microbiota composition also extend to other physiological functions, including the inhibition of bile acid secretion (Hu et al., 2022), suppression of bicarbonate secretion (Xu et al., 2022) and the inhibition of immune surveillance (Brosnahan et al., 2014). Similar studies have also highlighted that morphine enhances the toxicity of Pseudomonas aeruginosa (Babrowski et al., 2012). Representative Gram-negative bacteria in the gut microbiota, such as those belonging to the genera Bacteroides, play an important role in ameliorating gut inflammation and maintaining the homeostasis of intestinal epithelial cells through the production of SCFAs (Smith et al., 2013). Prolonged treatment with morphine can lead to a reduction in Bacteroidetes, consequently giving rise to intestinal inflammation and compromising the integrity of the intestinal epithelial barrier (Kang et al., 2017). Another illustrative example lies in the Gram-positive Firmicutes phylum (Lactobacillales), which mitigates intestinal inflammation through butyrate salts and regulatory T cells (Furusawa et al., 2013). Prolonged morphine treatment also leads to a significant reduction in the abundance of the Firmicutes phylum (Kang et al., 2017). Interestingly, naloxone (a classic antagonist of opioid stimulants) can effectively alleviate the reduction of thick-walled microbial organisms caused by prolonged morphine treatment (Meng et al., 2015). In another study, naloxone was also shown to normalize morphine treatment-induced gut dysbiosis (Wang et al., 2018; Essmat et al., 2023). This could mean that the dysregulation of gut microbiota caused by opioid drugs can potentially be alleviated through the judicious use of peripheral μ-opioid receptor antagonists, thereby reducing the side effects associated with opioid medications.

Recent research has revealed that morphine treatment induces an enrichment of bacteria producing β-glucuronidase in the mouse gut. This leads to the accumulation of chemotherapy drugs in the intestine, culminating in the onset of gut dysbiosis (Meng et al., 2023). For cancer patients undergoing long-term opioid treatment for pain management, the development of interventional drugs targeting the gut microbiota associated with this treatment could potentially mitigate the adverse effects of chemotherapy. In addition, an elevated abundance of Enterobacteriaceae is frequently considered as an indicator of gut dysbiosis (Rizzatti et al., 2017). Recent studies have confirmed that Enterobacteriaceae are markedly enriched in histological samples of inflammatory bowel disease (Gevers et al., 2014). Prolonged exposure to morphine results in a significant increase in the abundance of Enterobacteriaceae (Kang et al., 2017). Furthermore, disparities may exist in secondary intestinal microbiota, such as filamentous bacteria (Earley et al., 2023). Additionally, there have been reports suggesting that the presence of certain less abundant genera, such as Verrucomicrobia and the genus Akkermansia, may also be augmented under the influence of opioid analgesics (Kim et al., 2021; Abu et al., 2022).

The results of animal experiments regarding changes in gut microbiota species due to opioid use are robust and compelling. However, two remaining questions remain concerning the translatability of these findings to humans. First, the use of opioids in animal studies involves high doses and short treatment durations. Second, there is a notable discrepancy in the presence of 85% of secondary phyla between the murine gastrointestinal tract used for preclinical experiments and the human body (Hu et al., 2024). Whether these subtle differences will influence the outcomes of clinical experiments remains to be investigated. Encouragingly, although the data are limited, there have been reports in human studies that parallel those from animal research. Notably, these include a decrease in Ruminococcaceae (Acharya et al., 2017) and an increase in Enterobacteriaceae (Gicquelais et al., 2020).

In recent years, there has been growing interest concerning the mechanism by which opioid drugs influence the gut microbiota. The latest data has forged a connection between opioid treatment and gut microbiota translocation. Several preclinical studies have corroborated that classical opioid drugs, including morphine (Meng et al., 2015; Wang et al., 2018; Zhang et al., 2019a) and hydromorphone (Sharma et al., 2020), can disrupt intestinal epithelial integrity and barrier function in mice. This disruption ultimately leads to bacterial translocation and gut inflammation. On one hand, opioids impede gastrointestinal motility and substance transport by binding to the opioid mu-receptors in the myenteric plexus of the gut (Goudsward et al., 2024), This impairment in bowel motility can lead to the overgrowth of harmful gut bacteria, potentially resulting in gram-negative sepsis (Meng et al., 2013). On the other hand, in a mouse model of sepsis, it has been shown that morphine treatment induces the dissemination of Gram-positive bacteria (Meng et al., 2015). The translocation of Gram-positive bacteria subsequently triggers an excessive expression of IL-17A and IL-6, ultimately compromis of intestinal epithelial barrier function and further exacerbating systemic inflammation (Meng et al., 2015; Kang et al., 2017). Similar conclusions have also been drawn from clinical data (Meng et al., 2015).

The displacement of gut microbiota induced by opioid medications can lead to disruptions in intestinal epithelial integrity (Banerjee et al., 2016) and trigger inflammatory immune responses (Thomas et al., 2022). We posit that future research might delve into the intricate interplay between gut microbiota and immune responses within the intestinal epithelial barrier. In animal models, it has been found that gut microbiota serves as a key determinant in initiating autoimmune T-cell responses (Berer et al., 2017). The gut microbiota is likely to influence the function of many innate immune cells, thereby actively participating in the regulation of immune responses (Santa-Cecília et al., 2019). Notably, research has demonstrated that morphine exhibits no discernible impact on the gut microbiota of mice with TLR2 knockout or immunodeficiency (Banerjee et al., 2016). This observation underscores the critical role of immune function in maintaining intestinal microbial homeostasis and preventing dysbiosis.

Opioids often exert a dual effect on immune regulation. They typically suppress the immune response of T cells and B cells (Roy et al., 2011). However, in the gastrointestinal tract, they can paradoxically activate the immune system, giving rise to inflammatory responses (Meng et al., 2013; Kang et al., 2017). Research has shed light on the key role of neuroimmune effects induced by opioids in the gastrointestinal tract. Morphine directly induce changes in intestinal epithelial function and compromise the integrity of tight junction proteins through TLR regulation (Banerjee et al., 2016). Evidence suggests that these neuroimmune effects actively contribute to maintaining intestinal epithelial barrier function (Talbot et al., 2020). However, prolonged opioid therapy inhibits immune responses (Banerjee et al., 2016), ultimately leading to dysbiosis and an altered composition of the gut microbiota. This, in turn, leads to the disruption of the intestinal epithelial immune barrier and the development of analgesic tolerance, presenting a significant challenge in opioid management.

Opioids represent the most commonly prescribed medications for pain management. However, their prolonged usage carries inherent risks including the development of analgesic tolerance, potential overdose, and even mortality. In cases of NP, the downregulation or decreased sensitivity of opioid receptors frequently leads to diminished effectiveness of opioids, resulting in opioid tolerance (Balogh et al., 2019; Li et al., 2023). The prevailing view posits that alterations in opioid receptors are the primary drivers of opioid analgesic tolerance. These changes encompass a variety of mechanisms, such as the downregulation of spinal opioid receptors (Balogh et al., 2019), desensitization stemming from adaptive modifications within opioid receptor cells (Bai et al., 2020), and the functional modulation of cell membranes resulting from the repeated use of opioid analgesics (Kliewer et al., 2019). In addition, the mechanism of opioid-induced tolerance to pain relief is a topic of ongoing debate and investigation. Several potential mechanisms have emerged from relevant studies, including immune activation and neuroinflammation (Deleo et al., 2004; Hutchinson et al., 2011; Wang et al., 2012), receptor desensitization and downregulation (Balogh et al., 2019), upregulation of protein kinase A, and the release of norepinephrine and acetylcholine (Ma et al., 2001). Recent research has highlighted the significant role played by gut microbiota dysbiosis in pain modulation and tolerance through the activation of the gut-brain axis. Changes in the composition of gut microbiota have been shown to directly impact opioid tolerance in mouse experiments (Kang et al., 2017). Notably, the use of vancomycin alone, resulting in a reduction of gram-positive bacteria, prevented the development of opioid tolerance (Kang et al., 2017). While prior research on opioid-induced analgesic tolerance primarily focused on spinal mechanisms (Porreca et al., 1998), recent years have witnessed a growing emphasis on peripheral dysregulation mechanisms (Rivat, 2016; Zhang et al., 2016). Studies have revealed the pivotal role of the dorsal DRG in downregulating mu-opioid receptor (MOR) gene expression in the spinal dorsal horn, consequently diminishing the analgesic efficacy of opioids such as morphine (Zhang et al., 2016). The gut microbiota primarily contributes to the onset and progression of pain through its interaction with DRG and nerve endings within the intestine (Xu et al., 2022). Furthermore, recent research underscores the importance of the DRG in the development of pain tolerance (Jin et al., 2023). Another study has revealed that the gut microbiota is involved in the development of morphine tolerance in primary afferent neurons through its interaction with tetrodotoxin-resistant (TTX-R) sodium channels (Mischel et al., 2018). These research findings confirm the important role of gut microbiota in the peripheral analgesic mechanism of opioids.

Furthermore, a substantial body of evidence supports the key role of neuroinflammation in the development of morphine tolerance (Trang et al., 2015; Gei et al., 2024; Wang et al., 2024). Within neuroinflammatory responses, microglia have long been recognized as important markers for observation. Laboratory findings indicate that the elimination of microglia can effectively prevent opioid-induced pain hypersensitivity (Burma et al., 2017). In addition to its role in the analgesic tolerance process, glial activation actively participates in the development of NP (Gei et al., 2024) and is a key element in the mechanisms related to opioid treatment (Martínez-Navarro et al., 2019). MOR stimulation can lead to the activation of microglia (Ferrini et al., 2013). Interestingly, this opioid-induced glial response may counteract some of the analgesic effects (Martínez-Navarro et al., 2019). Furthermore, specific metabolic products of gut microbiota, such as lipoteichoic acid and LPS, can also induce the activation of glial cells within the nervous system (Obata and Pachnis, 2016; Bhave et al., 2017). Once activated, glial cells release an array of cytokines into the internal environment (Leduc-Pessah et al., 2017), consequently facilitating the development of analgesic tolerance (Drossman and Szigethy, 2014). These reports suggest that reducing harmful gut bacteria, which are associated with increased pain tolerance, can effectively alleviate the development of pain tolerance.

Recent studies have indicated that a portion of morphine analgesic tolerance arises from a decrease in the levels of intestinal β-glucuronidase, resulting in reduced hydrolysis of morphine metabolites back into morphine (Wang and Roy, 2016). Compelling evidence indicates that specific bacteria in the gut, such as Bifidobacteria and Lactobacilli, produce β-glucuronidase (Zhang et al., 2019a). Moreover, further research has confirmed the association of Bifidobacterium and Lactobacillus with opioid tolerance. Probiotics enriched with these microbial strains have demonstrated their effectiveness in alleviating analgesic tolerance in morphine-treated mice (Zhang et al., 2019a). The observed changes in Bifidobacterium and Lactobacillus are particularly intriguing, as these strains themselves contribute to maintaining intestinal homeostasis (Wang et al., 2014). Furthermore, recent investigations into opioids have revealed that FMT significantly reduces opioid withdrawal responses (Zhang et al., 2021). In summary, these studies underscore the substantial role of gut microbiota in opioid treatment, suggesting that supplementation with associated gut microbial strains may potentially prolong the analgesic efficacy of opioids.

Opioid addiction poses a significant challenge in the context of long-term opioid use for NP, often leading to varying degrees of withdrawal symptoms upon discontinuation of the medication (Simpson et al., 2020). Preclinical evidence strongly suggests that the gut microbiota actively influences behaviors associated with opioid addiction (Han et al., 2018; Lee et al., 2018; Hofford et al., 2021; Thomaz et al., 2021). Recent research has been exploring the impact of gut microbiota on opioid withdrawal. A recent preclinical study yielded results supporting the effectiveness of FMT and ABX treatment in reducing naloxone-induced withdrawal responses (Thomaz et al., 2021). Furthermore, a study identified significant gender differences in the relationship between gut microbiota and opioid addiction (Ren and Lotfipour, 2023). Among the abundant evidence, a key produced by the gut microbiota, SCFAs, plays a crucial role in cocaine reward behavior (Kiraly et al., 2016). In instances where gut microbiota diversity is inhibited by ABX, there is an increase in cocaine-conditioned place preference (CPP), a phenomenon that can be reversed through SCFA supplementation (Kiraly et al., 2016; Hofford et al., 2021). This suggests that diminished gut microbiota diversity, likely closely linked to SCFAs, instigates changes in opioid reward behavior. Research on SCFAs holds promise for the development of safer and more effective opioid treatment options.

Chronic morphine treatment activate the BDNF pathway through microglia, resulting in a reduction of dopamine-dependent reward behavior (Taylor et al., 2016). This underscores microglia as a key factor in the development of reward dysfunction induced by opioid drugs. Excessive activation of microglia in the ventral tegmental area leads to impaired brain reward behavior and diminished pleasure (Taylor et al., 2016), necessitating higher opioid doses for achieving reward and ultimately contributing to opioid addiction. Importantly, the gut microbiota can influence the morphology and activity of microglia (Gicquelais et al., 2020), consequently leading to reduced dopamine activity and disruptions in reward mechanisms (Taylor et al., 2016). Recent animal experiments have demonstrated that animals treated with FMT exhibited a significant restoration of normal reward behavior, accompanied by an observed increase in resting-state microglia (Lee et al., 2018). These research findings suggest that modulating the gut microbiota can exert a positive influence on the restoration of reward behavior and microglial function. Advancements in gut-brain axis research have suggested intriguing connections between gut microbiota, neuroimmune responses, and opioid withdrawal, offering fresh insights into opioid pain management. The development of gut microbiota-based therapies has introduced a fresh perspective to opioid withdrawal treatment and neuroimmune mechanisms. However, extensive clinical research is required to conclusively validate these possibilities and to further investigate the interactions between gut microbiota, opioid management, and neuroimmunity.

NP is often accompanied by a spectrum of depressive symptoms (Janakiram et al., 2023). Notably, the severity of depression tends to worsen in tandem with the intensity of pain (Agüera-Ortiz et al., 2011). Clinical psychologists often use sedative medication as an adjunct treatment during the process of treating depression. When considering the combination of opioid drugs and sedative drugs, particularly benzodiazepines, it is imperative to exercise a high degree of vigilance due to the potential risks associated with these medications (Liu et al., 2021). Previous studies have confirmed that when compared to using opioids in isolation for managing NP, the combination of opioids and sedative drugs may lead to a notable increase in the risk of drug overdose (Meyer et al., 2023). Furthermore, it can escalate to a critical level, potentially culminating in fatal respiratory depression (Liu et al., 2021). Research has shown that the activation of opioid receptors in the context of pain treatment can lead to or exacerbate depressive symptoms (Dagher et al., 2022). This pertains not only to patients already affected by mental health issues, but even to those deemed mentally healthy, long-term use of opioids significantly increases the risk of developing depression (Scherrer et al., 2016).

The gut microbiota plays a pivotal role in the development of depression by secreting neurotransmitters, including serotonin (Yano et al., 2015). The utilization of opioid medications can disrupt the gut microbiota, resulting in a depletion of serotonin levels and an augmented risk of developing depression (Parker et al., 2020). Notably, Lactobacillus and Bifidobacterium have been implicated in associations with depression and anxiety (Han and Kim, 2019). Their depletion can occur as a consequence of morphine treatment (Zhang et al., 2019a), potentially increasing the susceptibility to experiencing anxiety and depression during medication therapy. Encouragingly, specific strains of probiotics, such as Bifidobacterium, have demonstrated the capacity to mitigate depression by increasing BDNF levels and reducing oxidative stress (Messaoudi et al., 2011). Research has reported a decrease in BDNF levels in instances of depression, and clinical studies have shown that supplementation with probiotics can lower depressive symptoms (Jang et al., 2019). Another study on Bifidobacterium revealed that it, along with Lactobacillus, contributes to the reduction of depression levels by activating the vagus nerve and eliciting an immunostress response (Yang et al., 2024). Furthermore, research findings also suggest that increasing the abundance of the Ruminococcaceae family could also be a potential avenue for reducing depression levels (Zhu et al., 2019). A recent study suggests that Lactobacillus may help alleviate depression and anxiety induced by chronic stress by sustaining levels of interferon gamma (IFN-γ). Therefore, maintaining healthy levels of Lactobacillus or IFN-γ could potentially prevent or treat symptoms of depression (Clark et al., 2023). Although further exploration is warranted to fully understand the connection between depression and the gut microbiota, it is worth considering supplementation with specific gut microbiota, especially in cases necessitating long-term opioid medication use for NP treatment.

A clinical study has revealed that patients with higher depression scores often exhibit elevated levels of kynurenine, serotonin, and histamine (Han et al., 2022). These compounds are primarily produced by enterochromaffin cells under the influence of gut microbiota, ultimately contributing to the onset of depression. This also underscores the indirect influence of gut microbiota on the development of depression. In addition, another article highlights the serotonin-generating capability of Clostridium nexile, which demonstrates significant elevation in patients with severe depression (Williams et al., 2014). These clinical findings suggest a potential avenue for alleviating the depressive effects induced by opioid treatment, by targeting specific gut microbiota, such as Clostridium nexile, to modulate neurotransmitter production.