Various. The most common side effect of benzoyl peroxide is dryness. The dryness is usually mild, and may be countered with application of an oil-free moisturizer. Some patients may experience irritation when using benzoyl peroxide. If you experience mild irritation, try decreasing the frequency of applixation. If irritation is severe or persists, discontinue use of benzoyl peroxide.
Throw it away. Medications, both topical and oral, should be discarded if they develop a change in odor, color or consistency.
No. It whitens teeth and bleaches fabrics. Usual treatment for melasma is hydroquinone, often with tretinoin.