Background Metronidazole (MTZ) is indicated in the prevention of infections during surgical procedures. However, some data have shown that metronidazole has carcinogenic potential. Methods In the present work, we determined concentrations of metronidazole and its hydroxy metabolite (MTZOH) in colorectal cancer patients. MTZ and MTZOH were measured in tumor tissue and surrounding healthy tissue by LC-ESI-MS-MS method. Results We found different concentration of MTZ and MTZOH in colorectal cancer and healthy tissue. Interestingly, we noted a higher level of the above substances in women vs . men, both in healthy and cancerous gut. Conclusion We suggest that women are more exposed to a potential carcinogenic effect of metronidazole than men.