AbstractIntroductionMelasma is a chronic hyperpigmentation disorder, and its treatment poses a challenge to dermatologists due to its chronicity and resistance to conventional therapies. Oral isoniazid is used for the treatment of tuberculosis. One of us had previously showed that topical isoniazid exerts a strong depigmenting action in animal models. In this clinical trial, we assessed the therapeutic effect of topical isoniazid on melasma.MethodsTwenty female patients suffering from epidermal melasma were enrolled and divided equally into two groups. The treatment group received topical isoniazid 10%, and the control group received the cold cream vehicle as the placebo. All participants were advised to avoid sunlight and used SPF 50 sunscreen. Patients applied topical agents once daily at night for 3 months. The melanin and erythema indices were measured by colorimetric evaluations at baseline and after 4, 8, and 12 weeks of treatment. At these time points, the (mMASI) score was also determined, as was the subjective evaluation through Melasma Quality of Life Scale (MELASQOL) scores. Blood tests were performed to evaluate CBC and the liver enzymes.ResultsAll patients completed the 12‐week study. In the treatment group, a significant decrease in melanin index from 63.77 ± 6.27 at baseline to 55.92 ± 5.79 was recorded (p = 0.001). Very minimal clinical changes were also seen in the control group and melanin index was decreased from 62.65 ± 2.23 to 61.25 ± 2.34 (p = 0.004). Clinically significant differences were observed in the rate of changes between both groups. These findings indicate that topical isoniazid has significant depigmenting effects compared to the placebo (p = 0.001). The erythema index remained unchanged in both groups. In the treatment group, the mMASI score was 5.63 ± 3.28 at baseline and 2.13 ± 1.71 at the last follow‐up, significantly reduced compared to the control group (p = 0.002). The MELASQOL score indicated a significant improvement in the quality of life in the treatment group.ConclusionThis clinical trial shows for the first time that topical isoniazid is effective in treating melasma. Further clinical trials are necessary to confirm the efficacy and tolerability of topical isoniazid in comparison with other skin‐depigmenting compounds.