In this study, we assessed the impact of seven metformin targets on MTDS. The results indicate that metformin use does not reduce the risk of these cancers and may even increase the risk of colorectal cancer. To date, although many studies suggest that metformin use may reduce the incidence of MTDS, most of these studies are likely subject to biases and confounding factors [22], and their results should be interpreted with caution. Our study provides evidence comparable to that of a randomized controlled trial, which does not support the use of metformin for the prevention of MTDS.

The initial study linking metformin use to colorectal cancer risk emerged in 2004 [23]. Following this, numerous population-based studies, case-control cohort investigations, and meta-analyses have explored the connection between metformin use and CRC risk. The findings have been varied, with some studies indicating a reduced risk [24,25,26,27], others finding no significant association [28, 29], and a few suggesting an increased risk of CRC [30, 31]. For individuals with diabetes but not on antidiabetic medication, the incidence rates of colorectal cancer and hepatocellular carcinoma are increased. When using metformin, the incidence rates of colorectal cancer in women and hepatocellular carcinoma in men decrease to non-diabetic levels [25]. A meta-analysis incorporating four observational studies suggested that metformin treatment is significantly associated with a reduced risk of colorectal cancer in patients with T2DM [32]. However, a multicenter study indicated that metformin did not reduce the incidence of colorectal cancer in diabetic patients [28]. A large case-control analysis using the General Practice Research Database indicated that metformin use is not associated with a reduced risk of colorectal cancer, and long-term use may even increase the risk of colorectal cancer [30].

In fact, there is no consistent conclusion regarding the relationship between metformin and other MTDS. Surprisingly, a meta-analysis showed that metformin is associated with a roughly 70% reduction in the risk of hepatocellular carcinoma in patients with T2DM, but there was evidence indicating significant heterogeneity among the studies included by the authors [33]. However, another meta-analysis showed that the use of metformin is not associated with a reduced risk of hepatocellular carcinoma [12]. A meta-analysis of 7 cohort studies including 591,077 patients found that metformin therapy significantly reduced the incidence of gastric cancer compared to other treatments [34]. Additionally, a pooled analysis combining data from 3 case-control studies within the Stomach Cancer Pooling Project found no significant link between chronic metformin use and gastric cancer [35].

It is important to note that the majority of current research on metformin and cancer consists of observational studies, often utilizing historical medical records or insurance data, rather than being specifically designed to assess metformin’s impact on cancer. The data on dosage, duration, and temporal changes in metformin treatment, as well as other adjunctive therapies (including insulin, sulfonylureas, and other medications), are often incomplete. These studies are frequently affected by immortal time bias, selection bias, and other confounding factors, potentially leading to an overestimation of metformin’s benefits on cancer incidence and outcomes. While some randomized clinical trials have recently emerged to evaluate metformin as an adjunct therapy for various cancers, no definitive benefits on cancer have been demonstrated. A recent meta-analysis comprising nine randomized trials found that, compared to anticancer therapy alone, the use of metformin as an adjunctive anticancer treatment did not enhance tumor response or extend overall survival [36]. Recently, a large phase 3 placebo-controlled trial involving 3,649 women with high-risk operable breast cancer found no difference in invasive disease-free survival or cancer mortality between those who received adjuvant metformin and those who received standard therapy [37]. A phase 2 single-arm trial found that adjuvant metformin treatment for colorectal cancer achieved disease control in 41% of participants [38], higher than the control rates seen in previous studies with monotherapy [39]. However, evidence indicates that such trials are often subject to time-related biases and confounding factors [40, 41].

The mechanisms by which metformin might increase colorectal cancer risk are complex and multifactorial, reflecting its diverse biological effects. One critical factor is metformin’s influence on gut microbiota composition. While metformin-induced changes in microbiota have been associated with improved glucose homeostasis, they may also result in the overproduction of potentially carcinogenic metabolites, such as secondary bile acids and pro-inflammatory cytokines. These metabolites can disrupt the intestinal epithelial barrier, leading to chronic inflammation—a well-established driver of colorectal tumorigenesis. Additionally, metformin’s systemic glucose-lowering effects reduce circulating insulin levels, which is generally protective against cancer driven by insulin signaling. However, in colorectal tissues, metformin may exert paradoxical, localized effects by altering insulin signaling pathways. Specifically, metformin might enhance insulin resistance within epithelial cells, inadvertently promoting hyperactivation of the PI3K/AKT/mTOR pathway—a signaling axis frequently dysregulated in colorectal cancer [42]. This pathway is known to stimulate cellular proliferation and survival, creating a pro-tumorigenic environment in the colon. Furthermore, metformin’s effects on metabolic and inflammatory pathways may vary depending on tissue type. For example, in hepatocellular carcinoma, metformin appears to exert anti-inflammatory effects and inhibit cell growth via AMPK activation and mTOR inhibition [43]. However, in colorectal tissues, its influence on local inflammation and microbiota may override these protective effects, resulting in divergent outcomes. These findings align with prior studies showing variability in metformin’s effects across cancer types, highlighting the need for tissue-specific mechanistic studies. It is important to note that the mechanisms discussed here are speculative and based on existing literature. Our study is focused on causal inference through MR, and while we propose potential mechanisms, we did not directly test these biological processes. These mechanisms should be considered as hypotheses to guide future research. Direct experimental validation of these proposed pathways is necessary to fully understand the role of metformin in colorectal cancer and its broader implications for cancer prevention. Future research should integrate microbiome analysis, insulin signaling assays, and metabolic profiling to disentangle these dynamics. This will be crucial for optimizing metformin’s therapeutic use while mitigating potential risks in colorectal cancer [22].

Our study suggests that, in addition to its well-established role in glycemic control, metformin may also have an unintended effect on cancer risk—particularly increasing the risk of colorectal cancer. While metformin remains a cornerstone in diabetes management, these findings underscore the need for clinicians to be cautious in prescribing metformin for cancer prevention in patients with T2DM, particularly in those at risk for colorectal malignancies. The dual effects of metformin—both as a glucose-lowering agent and as a potential modulator of cancer risk—raise important questions for clinical practice. Given that patients with diabetes are already at higher risk for various cancers, particularly those of the digestive system, the potential pro-carcinogenic effects of metformin in certain tissues must be carefully considered. If future studies confirm that metformin increases the risk of colorectal cancer, alternative therapies or adjunctive treatments may be needed for patients with high cancer risk. Additionally, these findings suggest the importance of continued monitoring and personalized treatment strategies for diabetic patients, where cancer risk is an added concern.

Our study has several advantages. First, there are currently no well-designed large randomized controlled trials that investigate the relationship between metformin use and MTDS. MR studies can avoid time-related biases and confounding factors, providing evidence comparable to randomized controlled trials. Second, although some MR studies have explored the link between metformin and certain tumors, their selection of metformin instrumental variables was not rigorous, as they only used the UK Biobank population taking metformin as a basis [44]. The IVW method was used as the primary approach for causal inference due to its efficiency under the assumption that all genetic instruments are valid and free from horizontal pleiotropy. However, this assumption may not always hold, as unmeasured pleiotropy could introduce bias. To address this, we conducted complementary sensitivity analyses, including MR-Egger regression, weighted median, and weighted mode methods, which provide robust causal estimates even when some instruments are invalid. The consistency of results across these methods, coupled with the absence of significant pleiotropy indicated by the MR-Egger intercept test, supports the robustness of our findings. The genetic instruments used in this study were rigorously selected and validated through genetic colocalization and F-statistics exceeding 10, minimizing the risk of weak instrument bias. While the IVW method assumes no directional pleiotropy, the robustness of our findings across multiple methods provides confidence that pleiotropic effects are unlikely to drive the observed associations. Finally, we conducted a meta-analysis using different GWAS datasets, which enhances the persuasiveness of our study. However, our study has some limitations. First, the specific mechanisms by which metformin might increase the risk of colorectal cancer are still unknown. Second, the GWAS data we analyzed come from European populations, and the results may not be generalizable to other ethnic groups. Third, we were unable to conduct subgroup analyses, such as examining the effect of metformin on malignant tumors specifically in diabetic patients. Fourth, the datasets we utilized (GTEx, eQTLGen, Zheng et al., FinnGen, and EBI) provide summary-level genetic data, but do not include detailed clinical or phenotypic information for each participant. While demographic information such as age, sex, and ethnicity were available for most datasets, other important variables, including diabetes prevalence and comorbidities, were not consistently reported. Lastly, the lack of significant associations for cancers other than colorectal cancer could partly reflect limited statistical power due to smaller case numbers in the underlying GWAS datasets. For instance, cancers such as hepatocellular carcinoma and small intestine cancer had relatively few cases compared to colorectal cancer, potentially reducing the precision of causal estimates. However, the consistency of these null results across multiple sensitivity analyses and the robustness of our genetic instruments provide confidence that these findings are not artifacts of insufficient power. Future studies with larger GWAS datasets may provide further clarification of these potential associations.