Any drug has the potential to be dangerous. Paracetamol has the most number of overdose admissions in hospital than any other drug, according to my mother (a nurse for 30yrs). Anyone can be allergic to common pharmaceuticals e.g anti-biotic medicine. Personally I think tobacco and alcohol are most dangerous. And seeing some very close people to me suffering from schizophrenia due to cannabis use at an early age, and later methamphetamine's. From my personal experience, I consider pharmaceuticals, especially 'downers' (Xanax etc) with heroin and alcohol as the 3 most dangerous in terms of causes of death, especially when the 3 are combined.
Amphetamines and psychoactive drugs such as LSD and cannabis are more dangerous to the mental health of a person, then actual risk of death from my experience and research. Although they still can be deadly.
All drugs have a purpose, when used correctly! Pharmaceuticals taken at the right dose, under medical supervision is very safe. Drugs of addiction cause dependence and tolerance, leading to overuse, and or accidental overdose.
Illegal street drugs are very dangerous. There is no official guidelines to the manufacturing of the drug, and therefore absolutely no idea of the strength, quality, ingredients and overall procedure, which really means a gamble with your life. Which many are willing to take.