Pentobarbital

The core danger is a narrow therapeutic index; a modest overshoot in dose can suppress breathing to a fatal degree, and there's no reliable antidote comparable to naloxone for opioids. Mixing with alcohol, opioids, or other sedatives is extremely dangerous. Barbiturates cause tolerance and dependence, and withdrawal can be life-threatening with seizures. Not medical advice.

Two mechanisms account for nearly all of it. The first is additive central nervous system depression: with opioids, benzodiazepines, alcohol, sedating antihistamines or general anesthetics, respiratory depression and loss of airway reflexes are the danger, and barbiturates have a narrower margin than benzodiazepines because no equivalent of flumazenil exists to reverse them.

The second is enzyme induction. Pentobarbital induces CYP3A4, CYP2C9, CYP1A2 and glucuronidation, lowering exposure to warfarin, hormonal contraceptives, corticosteroids, doxycycline, protease inhibitors, ciclosporin and several antiepileptics; contraceptive failure is a documented consequence. Induction builds over one to two weeks and takes a similar time to fade once the barbiturate stops, so the loss of it can then push those same drugs into toxicity.

Barbiturates also induce delta-aminolevulinic acid synthase and can precipitate an attack in acute intermittent porphyria.

Checking a whole stack? Run it through interactions + stacks.