The best way to refuse is to just say "I'm good man" or something like that. Now if they keep trying to get you to do it then just leave...or do it[which i do not recommend considering most drugs are illegal when it comes to any use that isn't prescribed by a medical profesional such as your doctor or someone above him/her],but yeah it's best to just say no or leave.
if you brush your teeth both befor and after you eat you can avoid any type of infection in your mout sooooo even if you cant do this judt brush your teyh teice a day will help
