Did Shipman's victims, who died of a heroin overdose, suffer much?

Believe me, heroin is a fast acting pain killer if injected, and a large enough dose would knock you out in seconds. But most people who have no tolerance to heroin will experience nausea and vomiting on a relatively small dose, which would not be high enough for them to lose concsiousness. So if the doctor got the dose right, then the deaths would be painless. And if he's using heroin instead of other poisons, then that was probably his intention.