Background/Objectives: Pain is a growing public health concern worldwide, and the use of combinations of drugs can improve their analgesic effects while minimizing their adverse effects. Drugs such as metformin (antidiabetic) and melatonin (sleep regulator) have analgesic potential in combination. In this study, we evaluated the pharmacological interaction between metformin and melatonin when orally administered in a rat model, using the formalin test. Methods: Female Wistar rats (220–350 g) were injected with 50 µL of 1% formalin in the dorsal surface of the right hind paw. Formalin produces pain-related flinching behavior, and antinociception was evaluated as the reduction in this response. The percentage of the antinociceptive effect was determined after the oral administration of metformin (30–1000 mg/kg), melatonin (10–150 mg/kg), and their combination (MMC). To establish the nature of the interaction, isobolographic analysis was performed in a fixed-dose ratio (0.5:0.5), based on the effective dose 50 (ED₅₀) values for each drug: metformin (947.46 ± 242.60 mg/kg) and melatonin (126.86 ± 37.98 mg/kg). To evaluate the mechanism of action, the receptor antagonist for metformin compound C (dorsomorphin) for AMPK inhibition, MT1 and MT2 melatonin receptor antagonists (4-P-PDOT, luzindole), and an opioid antagonist (naloxone) were employed. The rotarod test was used to evaluate the safety profile of the combination. Results: The metformin–melatonin combination significantly reduced the number of flinches in the second phase of the formalin test. The theoretical ED₅₀ for the combination (ED₅₀ T) was 537.15 ± 122.76 mg/kg. Experimentally, the ED₅₀ (ED₅₀ E) was significantly lower (360.83 ± 23.36 mg/kg), indicating a synergistic interaction for the combination involving opioidergic pathways, MT2 receptors, and AMPK activation. Conclusions: Oral metformin–melatonin coadministration could provide a therapeutic alternative for inflammatory pain.