N is painless (though I would guess that it doesn't taste good at all) and assuming you're drinking a full bottle of 100 ml, it will work just fine. Keep in mind that you will have to minimize how much you eat beforehand, you should only eat a light meal in the hours before CTB. No, there are no permanent damage associated with the usage of N. (as @befree has said below; oxygen deprivation could cause permanent damage if you fail to CTB and wake up)