ODing: Breathing and consciousness

[wings of wax]

While Swim knows that respiratory distress can occur while conscious due to for example allergic reactions, he was wondering whether one looses consciousness before breathing stops because of drugs, like opiates.

For example, if someone was to OD on heroin, would he not be unconscious before breathing was so depressed that he stopped breathing? Or can someone actually experience stopping breathing from a drug overdose or combination while conscious?