2 General Principles of Drug Action DEFINITION OF DRUGS Aveybroaddeinitioaofadnugwouldineludealhemicalsotherhanodthataietliving s."If the affect heln on the body. the dru cal can be medicine and a poison depending on conditions of use and the person using it. Another definition would be"medicinal agents used for diagnosis,prevention,treatment of symptoms,and cure of de c ere ease.All dr ng mor tha dve shave the pote are appear verse drug effects appearing a ribed as toxic effects. CLASSIFICATION OF DRUGS Drugs can be classified according to various criteria: 1.By origin-sources of drugs Drugs may be obtained from 1.Plants 2.Animals 3.Minerals or 4.Microorganisms.The drugs may also be semisynthetic or synthetic compounds.The sources of drugs are summa- rized as follows: A.Synthetic.Most of the drugs in use today are synthetic in origin.Such drugs are chemically pure and it is easy to maintain supply line. Ex.:Aspirin,Paracetamol B.Natural.There are number of natural sources.They are: (@)Plants.A number of plant based dr Most of these don't have ant synthetic substitutes.Sv other used in formulations that are sold across the counter in several countries. (b)Animal.S me i che ugs continu to be derived from caus e the bersome and expensive.Ex. heparin,ym dotrophins (c)Microorganisms.Following the accidental discovery of penicillin from a mould in 1928 and its successful use in chemotherapy in 1940,a large number of antibiotics 3

 A very broad definition of a drug would include “all chemicals other than food that affect living processes.” If the affect helps the body, the drug is a medicine. However, if a drug causes a harmful effect on the body, the drug is a poison. The same chemical can be a medicine and a poison depending on conditions of use and the person using it. Another definition would be “medicinal agents used for diagnosis, prevention, treatment of symptoms, and cure of diseases.” Contraceptives would be outside of this definition unless pregnancy was considered a disease. All drugs have the potential for producing more than one response. Some adverse drug responses which are unavoidable are appearing at therapeutic doses are termed as side effects. Incontrast, adverse drug effects appearing at extreme drug doses are described as toxic effects.  
 Drugs can be classified according to various criteria: 1. By origin—sources of drugs Drugs may be obtained from 1. Plants 2. Animals 3. Minerals or 4. Microorganisms. The drugs may also be semisynthetic or synthetic compounds. The sources of drugs are summa￾rized as follows: A. Synthetic. Most of the drugs in use today are synthetic in origin. Such drugs are chemically pure and it is easy to maintain supply line. Ex. : Aspirin, Paracetamol. B. Natural. There are number of natural sources. They are: (a) Plants. A number of plant based drugs such as vincristine, taxol, digoxin, quinine, reserpine, ergotamine, ephedrine, colchicine etc. are still a part of standard therapy. Most of these don’t have any synthetic substitutes. Several other plant products are used in formulations that are sold across the counter in several countries. (b) Animal. Some modern drugs continue to be derived from animal sources because the synthesis of such chemicals is very cumbersome and expensive. Ex. : Gonadotrophins, heparin, insulin, thyroid extracts and enzymes. (c) Microorganisms. Following the accidental discovery of penicillin from a mould in 1928 and its successful use in chemotherapy in 1940, a large number of antibiotics 2 General Principles of Drug Action 3