Darkening of the penis within a month after taking levofloxacin may be caused by photosensitivity reaction of levofloxacin, physiological factors, and acanthosis nigricans. 1. Photosensitive reaction of levofloxacin: levofloxacin has photosensitive reaction, after taking this drug the skin is irradiated by sunlight, there may be skin erythema, blisters, blackening and other symptoms. 2. Physiological factors: high concentration of sex hormones in the parts of sex organs, abnormalities in sex hormones during development may lead to a large amount of melanin precipitation resulting in localized darkening of the skin. 3. Acanthosis nigricans: a disease characterized by deepening of skin color and papillary or velvety thickening, which can be manifested as local skin pigmentation and lead to darkening of the skin. There are many other reasons for the darkening of the skin of the penis, you should go to the hospital in a timely manner to find out the cause of the disease, under the guidance of the doctor, avoid unauthorized use of medication or treatment.