In this context alcohol would likely refer to vodka which is basically ethanol mixed with water. Mouthwash contains regular alcohol (ethanol) not denatured alcohol as this would render it poisonous which is illegal for anything meant to be ingested/inserted into body parts even if it's not supposed to be swallowed. That being said you don't want to snort mouthwash, it has way too many ingredients that could destroy either your sinus wall, your drug, or both. Think of that fierce Listerine sting in the morning... not a pretty sight. Also what kind of amphetamines are we talking about about? It's good to know 'cause not all are a good match for vodka (ethanol/water). Sounds pretty useless to just 'rinse off' the drug from your sinuses, ideally you would dissolve the drug into a proper liquid and inhale or spray the solution into your nose...