Acquired colour vision disorders may be caused by ocular, neurological or metabolic disorders, but they can also be drug-induced. In some cases, these disorders may precede or reveal the onset of severe and sometimes irreversible eye damage. The drugs implicated mainly include: phosphodiesterase type 5 inhibitors such as sildenafil; digoxin; anti-infectives including interferon alfa; ethambutol; metronidazole; and some antimalarials.