Barbiturates like Pentobarbital or 'N' are short acting, rapid onset. They work in higher doses as a respiratory deprassant. My understanding is thr dosage will either be enough to induce death, or simply a short sedation period. I find no medical literature pertaining to any brain damage, or other long term side effects from single dose administration. Chronic abuse of course is a different matter.
I think a dosage of 10g or higher is most certainly a guarantee of a peaceful way to CTB.