There really isn't a 100% guarantee with any method that is one of the reasons, especially for me, that makes us uneasy to take that final step, but I think as long as you have the additional meds I think the odds are pretty much in your favour. Many supposedly do succeed even with out the additional meds.
Dammed if you do, dammed if you don't.
I say this regarding the fact that SI will stop at nothing to stop you albeit deceptively by making you hold onto small reasons which results in procrastination in hoping things change long term but when they don't, you end up right back where you are, contemplating ctb with the options at your disposal not particularly suitable for making certain that it will be 100% guaranteed.
We also don't have the luxury of being medically supervised in our attempts, we're rather much on our own except through interactions like this. Yeap, even a former member who once attempted told me that she nearly ctb with SN even if she made a few mistakes and I've seen several other threads of users who could've ctb with this method had they not let si get the better of them thanks to symptoms like tachycardia or realizing what they had just done is something they may not be able to take back, regretful that they perhaps didn't give themselves time to weigh this decision against alternative options but others were just found even if they didn't alert anyone to help them.
Yesterday, I saw a comment on Reddit from a User who said their younger brother took SN, the Amazon SN to be specific. He ate dinner the night before, asked his sisters to play games with him before bed, was dead when his mother got him up for school in the morning. My presumption is that since he ate dinner from the night before, it means his fasting period may have not been longer than 8 hours but everyone's metabolism is different. The tragedy aside of this loss, point being, that he managed to do it in, sometimes ctb just requires that ability to forsake all reason to care about fears, repercussions or anything that'll weight heavy on the consciousness before an attempt. It isn't easy at all but it's different for how long it takes for everyone, if they ever get to that point.