Nail disorders are defined according to their appearance and the part of the nail affected: the nail plate, the tissues that support or hold the nail plate in place, or the lunula. The consequences of most nail disorders are purely cosmetic. Other disorders, such as ingrown nails, inflammation, erythema, abscesses or tumours, cause functional impairment or pain. The appearance of the lesions is rarely indicative of their cause. Possible causes include physiological changes, local disorders or trauma, systemic conditions, toxic substances and drugs. Most drug-induced nail disorders resolve after discontinuation of the drug, although complete resolution sometimes takes several years. Drugs appear to induce nail disorders through a variety of mechanisms. Some drugs affect the nail matrix epithelium, the nail bed or the nail folds. Some alter nail colour. Other drugs induce photosensitivity. Yet others affect the blood supply to the nail unit. Nail abnormalities are common during treatment with certain cytotoxic drugs: taxanes, anthracyclines, fluorouracil, EGFR, tyrosine kinase inhibitors, etc. Some drugs are associated with a risk of serious and painful lesions, such as abscesses. When these disorders affect quality of life, the benefits of withdrawing the drug must be weighed against the severity of the condition being treated and the drug's efficacy, taking into account the harm-benefit balance of other options. Various anti-infective drugs, including tetracyclines, quinolones, clofazimine and zidovudine, cause the nail plate to detach from the nail bed after exposure to light, or cause nail discoloration. Psoralens and retinoids can also have the same effects.