What about cross-tolerance with alcohol?
it's the same, Alcohol, benzos and opioids affects gaba receptors and therefore can slow down the efficiency of barbiturates
but it depends on many factors, how much/how long you've been taking it, what dosage of pentobarbital you're gonna take, etc
I saw an email of P.N where he answers someone that he has seen a lot of people regularly taking heavy doses of benzodiazepines or/and Opioids dying with a dose of 15g of pentobarbital without making any withdrawal beforehand
but I also saw an email from a well known Swiss organisation saying someone to stop all opioids/painkillers consumption a leat 3 weeks before the final day
I think no one knows for sure, even docs, I asked about it in a lot of places since I'm concerned but I never received a clear answer, no yes or no, just some maybe
and when I say a lot of places I'm talking about professionals people used to deal with it
at the end of the day it's up to you to decide if you want to take the risk or not, personnally I wont and will perform my withdrawal to put all the odds on my side