I'd agree with muie, but also would like to add that my experience with pentobarbital (also called Nembutal) was a strange one.
100mg one day, I would feel a bit relaxed. 100mg another day, I'd feel dizzy and lightheaded and not really with it. Then 100mg on another day I would not feel much relaxation at all and would in fact sometimes get agitation and start ranting to myself out loud. I went through 30 x 100mg Nembutal over the course of a few months, taking note of the effects. The effects range from 1 to 10 on an arbitrary scale. Sometimes it works, sometimes it doesn't. It's hit and miss. I took 200mg and just felt drunk, and the greatest irony is I couldn't fall asleep even if I felt so relaxed because I was talking to myself like a maniac - that was until I took 90mg mirtazapine and that eventually got me to sleep - and it works well with Nembutal because it seems to synergise with the H1 receptor antagonism that mirtazapine is famous (or infamous, if you will) for.
Does anyone know what's behind these mixed reactions? Barbiturates are not my cup of tea, but I do believe they're the strongest sleep drugs because for me they've rarely failed to get me to sleep, especially the pheno (funny that), and of course, let's not forget, they are PERFECT for suicide, which is why the fast acting ones are no longer around - well, not to most people anyway.
Phenobarbital to me is actually stronger than pentobarbital despite it's slower onset and it's reputation as being weak is really not justified. It's damned strong stuff regardless of how your typical junkie thinks of it. If I take 240mg phenobarbital it begins to take effect in 30 minutes - seriously, that's all it takes to start hitting me, and within an hour I could be falling asleep on it. If I take 100mg pentobarbital, it takes effect in 15-20 minutes, but pento doesn't seem to really slam until about 90 minutes after I've taken it, whereas pheno slams me in 60 minutes and it slams hard as a brick wall. The only difference between these two drugs is the gradient of effect. They both get you to the same point, but the gradient is different. With pheno it's a slow onset then all of a sudden WHAM, you hit a brick wall and you are on your arse! With pento it begins it's effect almost immediately and initially it's weak but then ramps up gradually until you feel the stronger effects. Maybe that's why junkies prefer the gentler gradient of pento (if I say so myself). Maybe I'm not your typical cookie-cutter insomniac. I'm a dyed-in-the-wool insomniac, I never had trouble sleeping as a child, but as an adult getting to sleep can be a fucking nightmare.
Pento lasts about 8-10 hours, makes me feel a bit sleepy for about an hour or two after waking up. Pheno lasts a day and sometimes will kick my arse for 2 days. Pento is a very strong anxiolytic but not a very strong hypnotic for me. Pheno is both a strong anxiolytic and an enormously strong hypnotic. I don't understand why a drug that is classed as weak works so well for me - it's more reliable at knocking me on my arse than Nembutal. I know junkies prefer a quick strong effect, but pento doesn't give me such an effect unless I drop like 300mg and then it's lights out for about 6 hours and waking up with a very shitty hangover and wishing I had died - not unlike alcohol - where I want to lash out at anyone who irritates me, with feelings of paranoia and anxiety coming on hard once the drug wears off and then spend much of the day complaining about the weather, people, certain things, etc and being a pain in the arse to myself and everyone around me. I never got that with pheno. Strange but true. Pento hits quickly and gently and gets nice and strong as it goes from the 30 minute mark to the 60 minute mark, but it stops bluntly and brutally. Pheno is slow to hit, and starts to hit at 30-40 minutes, but when it gets to the 60 minute mark it seems to start hitting like a freight train, sudden and forceful, but it tapers off nice and gentle, and doesn't leave me with a hangover that makes me want to do nasty things.
Pentobarbital is like a straight 45 degree line, constantly increasing in effect to the peak, whereas phenobarbital is like a parabolic curve that seems to do nothing for a while then starts to ramp up like crazy.
BTW as far as toxicity is concerned, I think hexobarbital was the most toxic commonly available barbiturate, and not pentobarbital as many have claimed. Phenobarbital is less toxic due to it's lower lipid solubility but if enough of it is taken an overdose can easily be fatal.
