What are fatal over the counter drugs?

Many can be fatal,some of the most fatal are the semi synthetic opiods or opiate pain killers such as Fentanyl,buprenorphine,oramorph,morphine etc if take by patients who have never taken opiates or if taken in overdose.The opiates have the ability to knock a patient unconcious and to slow down their breathing until they die,basically due to lack of oxygen getting into the blood.The majority of prescription drugs in overdose can be fatal,thats one of the main reasons they are controlled either by being prescription only or as controlled drugs/substances.