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Non-genetic causes

Physical trauma can cause color blindness, either neurologically – brain trauma which produces swelling of the brain in the occipital lobe – or retinally, either acute (e.g. from laser exposure) or chronic (e.g. from ultraviolet light exposure).

Color blindness may also present itself as a symptom of degenerative diseases of the eye, such as cataract and age-related macular degeneration, and as part of the retinal damage caused by diabetes. Vitamin A deficiency may also cause color blindness.

Color blindness may be a side effect of prescription drug use. For example, red–green color blindness can be caused by ethambutol, a drug used in the treatment of tuberculosis. Blue–yellow color blindness can be caused by sildenafil, an active component of Viagra. Hydroxychloroquine can also lead to hydroxychloroquine retinopathy, which includes various color defects. Exposure to chemicals such as styrene or organic solvents can also lead to color vision defects.

Simple colored filters can also create mild color vision deficiencies. John Dalton's original hypothesis for his deuteranopia was actually that the vitreous humor of his eye was discolored:

I was led to conjecture that one of the humours of my eye must be a transparent, but coloured, medium, so constituted as to absorb red and green rays principally... I suppose it must be the vitreous humor.

An autopsy of his eye after his death in 1844 showed this to be definitively untrue, though other filters are possible. Actual physiological examples usually affect the blue–yellow opponent channel and are named Cyanopsia and Xanthopsia, and are most typically an effect of yellowing or removal of the lens.

The opponent channels can also be affected by the prevalence of certain cones in the retinal mosaic. The cones are not equally prevalent and not evenly distributed in the retina. When the number of one of these cone types is significantly reduced, this can also lead to or contribute to a color vision deficiency. This is one of the causes of tritanomaly.

Some people are also unable to distinct between blue and green, which appears to be a combination of culture and exposure to UV-light.