Black toenails are often caused by a fungal infection called onychomycosis. Fungal nail infections are treated with oral antifungal drugs and medicated nail polishes or creams, according to Mayo Clinic. In some cases, the nail is surgically removed or treated with laser and light-based therapies.
It may take up to 12 weeks to completely get rid of discoloration caused by fungus, because the discoloration is permanent and only improves as the nail grows out. To remove discoloration from the nail, clip away the affected free edge until the nail grows long enough for the entire area of discoloration to be clipped off.