Skin darkening around my chin, upper lip, cheeks, neck, and underarms. What is the cause? (photo)

Thank you for providing detailed background information. Skin darkening (hyperpigmentation) in areas such as the chin, upper lip, cheeks, neck, and underarms, particularly in someone who is prediabetic and taking metformin, raises several possible diagnoses.

Most Likely Causes:
Acanthosis Nigricans:

Common in individuals with insulin resistance or prediabetes.

Presents as dark, thickened, velvety patches especially on the neck, underarms, and facial folds.

It can sometimes extend to the cheeks and jawline in more diffuse cases.

Post-Inflammatory Hyperpigmentation (PIH):

Can occur after acne, irritation, or the use of topical treatments like hydroquinone, especially in those with medium to darker skin tones.

If hydroquinone caused a purplish hue, this may suggest exogenous ochronosis, a rare side effect seen with prolonged use or high concentration of hydroquinone.

Melasma:

A hormonal and sun-sensitive pigmentation disorder, frequently affecting the upper lip, cheeks, and forehead.

Can be worsened by UV exposure, heat, and hormonal fluctuations, and may persist despite treatment with TXA and glutathione if those triggers continue.

Drug-Induced Hyperpigmentation:

Though metformin is not commonly associated, some systemic treatments can rarely contribute to pigmentary changes indirectly through hormonal or metabolic effects.

Management Recommendations:
Gentle Skin Care Regimen: Avoid irritating agents (like strong acids or high-concentration hydroquinone). Use gentle cleansers and fragrance-free moisturizers.

Sun Protection: Daily broad-spectrum SPF 50+ sunscreen is critical. Even incidental exposure (like through windows) can worsen pigmentation.

Topical Alternatives:

Consider safer lightening agents like azelaic acid, kojic acid, niacinamide, or cysteamine cream.

Discontinue hydroquinone if ochronosis is suspected.

In-Office Procedures (after stabilization):

Consider low-fluence lasers, chemical peels, or microneedling under dermatologic supervision.

Given your history, the combination of insulin resistance, possible ochronosis, and hormonal factors may be contributing. A repeat dermatologic evaluation, potentially with a biopsy or Wood’s lamp exam, may help confirm the diagnosis and guide a more effective treatment plan.