Hi there is something wrong here with this information, or at least something is unclear to myself.
Alcohol is a depressant and causes the pupils to constrict. This is also true with opiate drugs, and benzo anti-anxiety drugs. NOT dilate. Dilate is when the pupils get bigger. Upper drugs will cause this dilation such as cocaine, meth, e, and hallucinogenic drugs such as LSD.
The easiest method for Police to check if a person is intoxicated at a checkstop is to shine a flashlight in the eyes of the person to check there pupils and whether or not their eyes react to the light.
Alcohol withdrawal on the other hand causes the exact opposite effect and there will be a slight dilation in the pupils. I believe it depends on how much the person had to drink and how much tolerance they have to alcohol. So this effect may vary.
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