I don't know how bad are opioid overdoses, though my understanding is that for at least some products, the opioid is mixed with paracetamol, specifically in order so you die from OD-ing on the paracetamol before you get high on the opioid.

As for stimulant medication, I haven't heard of this being a real danger - not at the doses that prescription drugs have. Hell, I personally know a person who ate two boxes (i.e. 60+ tablets) of methylphenidate in a suicide attempt, and all they got from it was couple hours of hyperactivity, followed by a big headache, after ER flushed their stomach; they got discharged less than 24 hours later. But sure, if you drop the pharmaceuticals and go for illegal drugs, where the doses can be 100x as high, I guess you can mess yourself up pretty bad and rather quickly.