From my research , as long as you can keep it down for at least 5-10 minutes, you will have absorbed enough so even if you vomit it doesn't matter. However if vomiting occurs that's why it's recommended to have the back up glass! Remember 2.5-3 grams is the Fatal dose usually for someone of average weight! So the key is to make sure your body has absorbed enough within that 5-10 minutes, and by fasting and taking the extreme amount of 25grams, it won't take long for fatality to occur. By following the guide I think most failures only occurs by SI kicking in and people calling for help xx :) (but I'm not an expert!!)