Alcohol or any other drug which reduces the supply of oxygen in the blood stream can impair the sensitivity of the cones which in turn

 Alcohol or any the drugs that can impair with the sensitivity of the cones which in turn reduces the visual activity or sharpness.
There are three types of photo-receptors are present in the retina of the eye that are rods, photosensitive retinal ganglion cells and cones. Cones have the property for vision. Alcohol and other drugs reduces the oxygen supply in the blood stream and impair the sensitivity of cones that's how they can reduce the vision.

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