Not everyone who partakes of a “lethal dose” of a drug dies.
The obverse of this would be: can someone die of a sub-lethal amount of a drug and still be considered to have died of the drug? I think (and others agree) that the answer is yes.
Consider this hypothetical. An individual partakes of an adrenergic-like drug that acts toxically on the vasculature. That drug causes vasospasm and/or vasoconstriction of cardiac blood vessels starving the cardiac tissue (muscle and electrical) of oxygen and “energy/food”. This in turn causes electrical instability and irritability, as well as poorly functioning musculature.
Next “stress” this individual with a fear of loss of integrity/life through fear of injury or capture. I don’t know; maybe have him chased by a wolf. This stress causes the body to churn out adrenaline as a natural reaction. In this hypothetical that adrenaline would compound the effect of the recently consumed drug on the heart. In addition to the fear stressor, add that the man runs say 100-200 yards. This compounds his heart’s problem by increasing demands for oxygen and energy supplies to the heart tissue. And on top of this he develops lung perfusion/ventilation mismatch in his lungs because he isn’t anywhere near fit and maybe gets a little bronchospasm in the mix because his lungs aren’t in great shape either.
He stops (wolf gone?) and tries to recover, but the insult to his system, his heart, is too great. A dysrhythmia develops, syncope and death result.
Did the drug kill him? It certainly set the stage. Was it a natural death? It was a dysrhythmia not necessarily directly related to the drug ingestion that actually took his life. Thank goodness we can ponder these things as hypothetical.