it has a big chance of back firing and you will probably choke on vomit which is very common
Although this is a risk, N (nembutal/pentobarbital) is fairly reliable as far as drug poisonings go. The risk of vomiting is lower than sodium nitrite which is commonly used here.
The issue is that there exist no suppliers for N at this time. There were 2 previous vendors that were confirmed to not be scams but they have since disappeared. Anyone claiming to sell N at this time is a scammer.
You are very unlikely to get a prescription for barbiturates as they have been replaced in medicine with benzodiazepines specifically because benzodiazepines are far safer (nearly impossible to die from on their own) and just as effective for sleep, reducing anxiety, etc.