In the vast majority of cases, one bottle (100ml @ 60mg/ml) of Nembutal

will be satisfactory. However, there is a small group (<1%) who may exhibit

a prolongued comatose phase before death (sometimes up to 24 hours).

Outright ‘failures’ remain extremely rare. Indeed, all of the reported failures

investigated were associated with discovery and medical intervention. This

emphasises the need for careful selection of the place chosen to take the

drugs.

In some of the analysed cases, the long comatose phase is associated with the

prolonged use of anti-psychotic medication, prior to taking the barbiturate.

It is presumed the induction of liver enzymes by these drugs causes some

degradation of the Nembutal before it reaches the brain. In this situation,

increasing the quantity of drug taken (eg. to 2 bottles, 12gm) may not

necessarily hasten the death.

(http://www.autoadmit.com/thread.php?thread_id=5875173&forum_id=2.#49945513)