The PPeH says,
“However, there is a small group (estimated at <1%) who may exhibit a prolonged comatose phase before death (sometimes up to 24 hours). [...]
“In some of the analyzed cases, the long comatose phase was associated with the prolonged use of anti-psychotic medication or chronic heavy alcohol use, prior to taking the barbiturate. [...]
“Exit has examined the use of several potentiating drugs which may be dissolved into the liquid [N], removing any possibility of extended coma. The most useful, Dilantin, (phenytoin sodium) is discussed in this chapter. [...]
“[N] of uncertain quality can have its potency enhanced by dissolving 1-2gm of phenytoin sodium in the solution before drinking.”
Are you sure there would already be Dilantin in it? I definitely want to avoid 24 hour coma.