What also seems to happen with these types of drugs, especially if they are combined, is that the user\patient forgets that they have already taken one dose and inadvertently takes another and then ODs. Some survive... not sure if they are the lucky ones or not... guess it depends upon the permanent brain\system damage incurred.
A co-worker lost his wife in October last year because of this very thing... different drugs including morphine.
He will tell you flat out it was her doctors that failed her... she had more than one... the doctors worked together.