Another consideration is that people have different body chemistries, and metabolize drugs differently. How the drugs were metabolized could depend on whether they were taken with food, or even the time of day they were taken.

Many medicines have side effects, including fuzzy thinking, and if he had already taken one and wasn't as clearheaded as usual, he may not have thought about the implication of mixing something else with it.

It sounds like a sad unfortunate accident to me.