Currently, diabetes mellitus type two is a public health challenge worldwide. Even though there are many oral hypoglycemic agents, a large part of the population continues to use herbal remedies with proven benefits. However, there are few works aimed at evaluating combinations of drugs and herbal remedies. These combinations of drugs and herbal substances can lead to a decrease in the therapeutic effect of each of them. The present work was designed to evaluate the combination of Metformin with aqueous extracts of chaya (Cnidoscolus aconitifolius) in a group of Long Evans streptozotocin-induced diabetes rats. Several combinations of aqueous extracts of C. aconitifolius and Metformin were tested and glycemia was measured in streptozotocin-induced diabetes rats. Additionally, the chemical profile of the extracts was determined by high performance liquid chromatography coupled mass tandem detector (HPLC-MS / MS). Results revealed that the combinations tested suggested an antagonistic effect between both compounds since the glycemia remained high in three of the four treated groups. Some of the compounds detected in chaya extracts by HPLC-MS/MS could give a clue of the explanation of this behavior. Conclusively, the therapeutic effect of Metformin may decrease when chaya is regularly consumed as a complementary herbal remedy, as used in a part of the Mexican population. It is recommended to deepen in the future in the pharmacodynamic part to explain this behavior.