Society makes me confused

Drugs, just what exactly defines a drug? According to Wikipedia; a drug is any substance that when absorbed by biological life that substance will alter normal bodily function. Going by that definition many foods should be considered drugs. A large number of the food-stuffs we eat today contain the flavour enhancer Mono-Sodium Glutamate, or the artificial sweetener Aspartame. Both of these chemicals have been linked to neurological damage according to several different studies. There has been much controversy around Mono-Sodium Glutamate (MSG). So much in-fact that it is now rarely listed on food ingredients as MSG, Nowadays they call it gelatin, calcium casinate, hydrolyzed vegetable protein, textured protein, hydrolyzed plant protein, yeast extract, glutamate, Autolyzed plant protein, yeast food or nutrient, glutamic acid, sodium casinate, autolyzed yeast, vegetable protein extract, senomyx. There are more names but the list goes on forever. So you see the term drug shouldn’t really have such a derogatory connotation because accord to the definition supplied in most dictionaries most food-stuffs will fit into that definition
Drugs are known for their effects they have on the human brain, but most drugs also affect the body too as the body and the brain are intimately related. Some drugs have been linked to mental illness such as Marijuana and its linkage to schizophrenia. Some people can be diagnosed with schizophrenia without using any such drugs and in so does that mean their evolution to schizophrenia was a normal process? In being a normal process does it then rule marijuana out of being classed as a drug? (As the definition of a drug is a substance that alters ‘normal’ bodily function)

Does anyone have any information that may help me understand something please let me know.