Onycholysis is the detachment of the nail plate from the nail bed. If drug-induced, it can be an isolated phenomenon, but it may also accompany or follow a cutaneous phototoxicity reaction due to drug intake and exposure to ultraviolet irradiation. Photo-onycholysis is a rare photosensitivity reaction due to exposure to either a natural or artificial source of light. Many drugs are responsible for this phototoxic reaction, especially tetracyclines, psoralens, chloramphenicol, non-steroidal anti-inflammatory drugs, fluoroquinolones, and, rarely, doxycycline. Any patient given enough of a therapeutic dose of an inducing drug and sufficient light irradiation can develop phototoxic reactions. While there is no need to avoid these drugs completely, precautions should be taken. Here we have reported the case of a patient who developed onycholysis of his fingernails with sparing of the toenails following administration of diclofenac therapy for lower back pain. The onycholysis was associated with a phototoxic reaction. The swellings resolved totally within two days, and the patient started to notice the separation of all fingernail plates from their nail beds. The patient was treated symptomatically. On follow-up, onycholysis had regressed slowly, and the condition recovered totally within three months without any sequelae.