It would depend surely on how the dose was titrated to the individual patient since you get a tolerance build up, so what would kill one person would not kill another. Perhaps someone medical can turn up to verify that, but my understanding is that's why so many ex addicts die when they return to drugs, they've lost their tolerance.
Shipman was convicted in January last year of murdering 15 of his elderly female patients by injecting them with a lethal dose of diamorphine.
Chemists' records show that during one three-month period, he prescribed 30mg injections of diamorphine to 12 different patients – doses strong enough to kill a fit and healthy person.