Depends if they are potent opioids & how dependent the person is on said opioids. Tho initially all opioids cause both a decrease in frequency & amplitude of peristaltic waves, a person taking opioids long term/regularly is less likely to be effected by this. Also opioids are CNS depressants (they act directly on the medulla, causing varying degrees of respiratory depression). So without more information (type of opioid, dose & frequency) they may delay things a bit initially, but as N is also a CNS depressant, it's possible there could be synergy between the opioid & Nembutal. IMO opiates would probably increase the respiratory depression. I can't imagine a situation where opioids would cause a paradoxical effect of barbiturates.