5mg IM. ETOH on board.
ETOH changes the while equation. A bit on how benzodiazepines work...
Benzo's work on the GABA receptor sites. That is NOT to say the directly activate the sites...you'll see why this is important in a second. A benzodiazepine increases the binding potential of the GABAa neurotransmitter, thereby increasing GABA activity and inhibitory action. Alone, this is VERY safe, as its action is limited by the amount of endogenous GABAa you have floating around. You'll get to a point where it doesn't matter how much more medication you give, there's not enough neurotransmitter to depress things anymore. However...
There ARE compounds that directly activate the GABA receptor, one of the more common ones being ethanol. As such if your intoxicated patient consumes a benzo, not only does he have his own GABAa to worry about, he's got a whole crapload of extra stuff that directly acts on that receptor floating around to knock him further towards a coma.
Most "benzodiazepine" ODs are really polysubstance ODs.