Zelgadis assumes that anyone hitting on him is exceedingly drunk by default, because otherwise... why? Zelgadis has more than a few issues, ones that haven't been resolved by realizing that he's not particularly odd-looking by local standards.

"How much have you been drinking that stuff?"

The tone in his voice suggests 'too much', but at least it is more of a 'stop it' than 'go away'.