When my was helping his wife's family clean out their attic, he came across diazepam for injection (IV/IM). I don't remember the details of the solution but it was pretty cool shit, burned like hell though, so I'm assuming it was in some kind of alcohol.
I would mix shots of oxymorphone with half diazepam half water. Never so much to endanger myself with a risk of overdosing and never alone. Good combo. That sudden heavy sedation was groovy.