1drug noun ˈdrəg

A drug is a substance designed by the western pharmaceutical industry, which is approved to be used as medicine.

: an illegal and often harmful substance (such as heroin, cocaine, LSD, or marijuana) that people take for pleasure.

Prescription of drugs by pharmeceutical Clinics:

There are drugs which are prescribed to patients by doctors, such drugs have regulated side negative effects, (negative side effects which can surface due to overdose, misuse or the consuming of such drugs after a long period of time in one’s life).

A drug is substance which contains pharmaceutical ingredients that can cause negative side effects to a human body after a process of time (Drugs leading to the negative side effects of violence and commiting crime)

The reason why pharmaceutic drugs are produced:

That which is designed for the purpose of treating diseases and for bringing in financial increase into the pharmaceutical industry.

A substance intended for use in the diagnosis, cure, mitigation, treatment, or prevention of disease (3) :

A substance other than food intended to affect and alter (change) the structure or function of the immune system in the body. 

Thus; making the immune system less resistant to certain environmental changes, so that the addiction towards pharmaceutical drugs may intensify.

The scientific field of pharmacology deals with the study of poisonous (toxic chemicals), and how to transform such chemicals into regulated drugs used for treating diseases. 
a obsolete : a substance used in dyeing or chemical operations

b : a substance used as a medication or in the preparation of medication
c according to the Food, Drug, and Cosmetic Act (1) : a substance recognized in an official pharmacopoeia or formulary (2) : 

: something and often an illegal substance that causes addiction, habituation, or a marked change in consciousness
— drug·gy also drug·gie adjective