Lorazepam has about 5-8x the half life of Ambien. So when you take an Ambien at night to fall asleep, by the next night when you want to drink, it will have been out of your system for hours. The Ativan, on the other hand, will STILL be in your system and with cause the alcohol to have a much stronger effect. I mixed the two of them once thinking I could handle it...and don't remember about 6 hours of the night.
If you take it only as needed and you are talking about the night after drinking, the alcohol + ambien + ativan does have potential to seriously depress your breathing and other vital functions. I doubt it would be lethal, but it isn't something I would really mess around with if I could avoid it. If you MUST drink, have a few less beers and ride the buzz for the last couple hours of the night before taking the ambien and ativan for sleep.
Edit- and also not feeling a buzz is part of alcohol's fun little trick where as the longer you stay drunk, the less drunk you feel, despite there being alcohol in your system. 12-15 beers in 6 hours equates to ~6-9 beers which is ~.12-.18 BAC. This means when you stop drinking your blood alcohol is still well above what is legally allowed as the driving limit. When you add the pills to it, even if you don't feel drunk, the alcohol is still in the mix to depress vital functions
Diagnosis: SAD, GAD, Bipolar II
Meds: Wellbutrin SR 2x 150mg/day, Klonopin 0.5mg 3x/day, Celexa 10mg PRN, Seroquel 50-100mg PRN
Failed Meds (so far...): Zoloft, Celexa (daily), Buspar
Stopped Meds: Atenolol 75mg/day, Ativan 0.5mg PRN