I just came across this via another thread here:
"Barbiturates are derivatives of barbituric acid and are classified as either short- or long-acting. The short-acting barbiturates (e.g., pentobarbital) when compared to long-acting agents differ by more rapid onset [...] have a shorter duration of action [...] . Pentobarbital possesses a duration of action of three to four hours and a variable elimination half-life of 15 to 48 hours."
If I read this correctly, pentobarbitol --aka N-- acts quickly, but doesn't last very long. Which would mean not only is a lethal dose of X amount, but you need to get that dose into your system rapidly, rather than spread out over time. So depending on how fast your body can absorb liquid, diluting X with whatever --beer, fruit juice, etc.-- could slow uptake of that dose and make it ineffective.
The information is via a link recently posted here: https://sanctioned-suicide.net/threads/a-good-n-case.7225/