Blood-glucose levels are at their highest typically one to two hours after eating a mixed meal. (A mixed meal is a meal that contains carbs, protein, and fat.) Protein and fat cause the carbs to digest slower. A mixed meal takes roughly four hours to finish digesting. During digestion, carbs are breaking down into glucose and entering the bloodstream. Normally, some of the glucose from the meal is packed away in the liver and saved, to be used later as needed.
When the meal is completely done digesting, the liver is supposed to release the glucose that was previously stored. Your body must always have glucose in the blood to keep the vital organs functioning properly.
Alcohol goes to the liver to be detoxified, processed, and broken down into safe byproducts. While the liver is breaking down the alcohol, it may not be able to release glucose normally. If the glucose release from the liver is compromised, then the insulin (or certain diabetes pills/medications) may continue to push the blood-glucose levels lower and lower.
A single drink can take two or more hours to be processed by the liver, so glucose regulation may be impaired for that amount of time or longer. The liver stays busy for two hours or more per drink, so the more drinks you consume, the longer you are at risk for low blood-glucose reactions. The figure helps clarify the concept.
The gray shaded area represents the rise and fall of the blood glucose after eating a meal. When foods are finished being digested and absorbed, the liver's job is to release glucose that had been previously stored. Alcohol impairs that process because the liver preferentially breaks down the alcohol. Hypoglycemia may ensue.
If you drink on an empty stomach, that means there is no carb digesting, so there's no glucose supply via digestion. Your liver is supposed to release glucose between meals. If alcohol impairs the liver from releasing glucose, you cut off your only glucose supply. Your meds can make your blood glucose drop too low.