**6. Conclusion**

As an old repurposed drug, metformin is inexpensive and generic and its research is thus carried out usually without industry support. Despite such challenges, it is heartening that overall preclinical and clinical results is overwhelmingly suggestive of a protective effect from metformin against various stages of a wide spectrum of cancers. Moreover, there are over three hundred registered clinical trials on metformin and cancer internationally as of mid-2020, of which approximately one third are actively recruiting. The trials involve metformin for pre-cancers, early stage as well as metastatic solid tumors, alone or in combination with other interventions including chemotherapy, radiotherapy, hormone therapy, immunotherapy (ICIs), targeted agents, statins, aspirin, doxycycline, nelfinavir, melatonin, disulfiram, vitamin C, diet in diabetics and non-diabetics. We thus look forward for the further establishment of metformin as an ideal repurposed agent for cancer prevention and treatment.
