What effect does caffeine have on ADH levels?

Caffeine inhibits ADH causing the levels in the body to lower.

Anti-Diuretic Hormone is released when the body wishes to conserve water. The hormone tells the kidneys to reabsorb more water which results in urine becoming concentrated.

Alcohol has a similar effect which is why, when it inhibits ADH, the body no longer attempts to conserve water. This results in frequent urination which eventually becomes clear as less waste has accumulated in the urine. The following morning after heavy drinking, the body is dehydrated resulting in concentrated urine. Coffee and caffeinated beverages are the worst hangover cures for this reason and will only starve the body of water further - resulting in a prolonged hangover!