## 2 The Influence of Different Factors on Oral Microbiome
### Medication Use and Probiotic Consumption  
Medications such as antibiotics, antimycotics, corticosteroids, methotrexate, and immunosuppressive agents can significantly disrupt the microbial balance in the oral cavity. The dysbiosis and immune suppression caused by medications contribute to chronic inflammation and microbial imbalances, key factors in the pathogenesis of OPMD and OSCC. Understanding these effects is crucial for mitigating risks in patients requiring long-term medication use [130, 131, 132, 133, 134]. Antibiotics, for example, can reduce microbial diversity by eliminating both harmful and beneficial bacteria, potentially leading to overgrowth of resistant strains and opportunistic pathogens [130, 131]. Corticosteroids and immunosuppressive agents, used to manage inflammatory and autoimmune conditions, can suppress the immune response, thereby affecting the oral microbiome. These medications can create an environment that favors the proliferation of opportunistic pathogens, increasing the risk of oral infections such as candidiasis [132, 133]. Methotrexate, a common immunosuppressive drug, has been shown to alter the oral microbiome by reducing the abundance of certain bacterial species and promoting the growth of others, leading to a dysbiotic state [134]. On the other hand, emerging studies suggest that certain drugs and targeted therapies may improve the oral microbiome and contribute to disease management. For example, antifungal agents like flucanazole have demonstrated efficacy in reducing _Candida albicans_ colonization, mitigating inflammation, and improving oral health outcomes in patients with oral candidiasis [135]. Furthermore, prebiotic compounds, such as inulin and fructooligosaccharides, have been shown to enhance the growth of beneficial bacteria while suppressing pathogenic species, potentially aiding in the management of dysbiosis-related conditions [136]. Immunomodulatory agents, including IL-17 inhibitors, have also been explored for their ability to reduce inflammation and restore microbial balance in periodontal diseases as well as autoimmune conditions affecting the oral cavity [137; 138]. These developments highlight the potential of integrating therapeutic strategies targeting the oral microbiome into clinical practice for both prevention and treatment of oral diseases such as OPMD and OSCC. Recent probiotic use also impacts the oral microbiome. Probiotics are beneficial bacteria that, when introduced into the oral cavity, can help restore microbial balance and improve oral health. Probiotic strains such as _Lactobacillus reuteri_ and _Bifidobacterium_ species have been studied for their ability to reduce the levels of periodontal pathogens and promote a healthier oral microbiome [139]. These beneficial microorganisms not only counteract harmful biofilms but also help modulate the host immune response, reducing inflammation and supporting tissue health. Large doses of commercial probiotics (\(\geq\)10\({}^{8}\) CFU mL\({}^{-1}\) organisms per day) taken for 2-6 months can significantly alter the oral microbiome, promoting beneficial bacteria but potentially masking disease-related microbial changes [140; 141]. The use of probiotics has shown promise in managing conditions like gingivitis, periodontitis, and even haltitosis by enhancing the beneficial microbial population and inhibiting the growth of harmful bacteria [142; 143]. Beyond reversing dysbiosis, probiotics play a crucial role in host modulation and biofilm control, highlighting their therapeutic potential in managing inflammatory oral diseases, including periodontal and peri-implant conditions. Probiotics can inhibit pathogenic biofilm formation, enhance the host immune response, and restore microbial balance, thereby mitigating inflammation and tissue destruction associated with these diseases, therefore possibly contributing to oral carcinogenesis [144].