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.

Inclusion Criteria: Latin American women aged 21 years or older, diagnosed epidermal or mixed melasma

Exclusion Criteria: pregnancy, breastfeeding, use of hydroquinone or tretinoin within 2 months before the study, or undergoing laser treatment, peels, microdermabrasion, or microneedling within 2 months of the study. Patients with a history of renal or liver disease, metformin allergy, diabetes, hypoglycemia, or those on medications that could interact with metformin will also be excluded.

De-identified individual participant data (mMASI scores, Global Melasma Severity Assessment, and reported adverse events) will be shared with researchers upon request. Data will be available after study completion, pending approval from the principal investigator and institutional review board (IRB). Access will be granted through a secure repository or data-sharing agreement.

IPD and supporting information will be available starting 6 months after study completion. Data will be accessible for 5 years following publication of the primary study results.

De-identified individual participant data (mMASI scores, Global Melasma Severity Assessment scores, and reported adverse events) will be shared upon reasonable request. Access will be granted to qualified researchers affiliated with academic or healthcare institutions, subject to approval by the principal investigator and a data-sharing agreement. Requests should be submitted via email to the principal investigator, and approved researchers will receive access through a secure repository.