I’m pretty sure no amount of alcohol is safe. I remember before college (in other words: before all the binge drinking.) I felt bad weeks after I had just a glass of wine.
Ethanol metabolizes to acetaldehyde, which is a chemical that causes several unpleasant symptoms. Among these is the so-called "Asian blush". (Asians have a higher incidence of acetaldehyde dehydrogenase deficiency.)
Any amount of alcohol you drink in excess of your liver's ability to express acetaldehyde dehydrogenase will cause damage. If you get a hangover, it was too much.
This isn't the same as being unsafe though, is it?
For me: safety means not causing permanent damage.