The goal of this clinical trial is to learn if metformin can treat melasma in Latin American women. The main question it aims to answer is: \- Can metformin help reduce the dark patches of melasma? Researchers will test two different doses of metformin (500 mg and 1000 mg) to see if either one helps improve melasma. Participants will: * Take a metformin pill every day for three months. * Go to the clinic twice: once at the beginning and once at the end of the study. * Get a phone call from the researchers to check how they're doing and ask about any changes or side effects.