GENERAL PRINICPLES OF DRUG ACTION 7 can be viewed from different perspectives,namely,the site of action and the general nature of the drug-cell interaction. 1.Killing foreign organisms.Chemotherapeutic agents act by killing or weakening foreign organisms such as bacteria,worms,and viruses.The main principle of action is selec- tive toxicity.i.e.the drug must be more toxic to the parasite than to the host. 2.Stimulation and depression.Drugs act by stimulating or depressing normal physi ological functions.Stimulation increases the rate of activity while depr ession reduces it. 3.Irritation.It is a non-specific action of a drug tha e body tissue 4.Replacement.Drugs serve as replacement of essential body chemicals that are ei- ther absent or present in less than required quantity due to disease.Ex:Insulin is used in diabetes.Levodopa therapy in Parkinson's disease. MECHANISM OF DRUG ACTION A drug act by virtue of its various properties like physical,chemical,physiological etc.The fundamental mechanisms of drug action can be distinguished into following categories. 1.Physical Properties A physical property of the drug is responsible for its action (ii)Mass.By increasing the bulk of drug in intestine produce laxative effect.Ex:Isapgol (i)Adsorption.Certain drugs like kaolin adsorb water on to its surface and there by motility (iv)Radioactivity.The radioactive substances are commonly used to treat cancer.Ex:125. 2.Chemical Properties The drugs react extracellularly according to simple chemical reactions like neutraliza- tion,chelation,oxidation ete.Ex: (i)Aluminium hydroxide neutralizes acid in stomach (ii)Toxic heavy metals can be eliminated by chelating agents like EDTA,BAL, penicillamine ete. (ii)Oxidising agents are germicidal. 3.Through Enzymes E阳 zyme s are very influence of enzymes.Drugs may either increase or decrease (i)Adrenaline stimulates adenyl cyclase (ii)Pyridoxine acts as a cofactor and increases decarboxylase activityGENERAL PRINICPLES OF DRUG ACTION 7 C-8—N-CHEMI\CHE2-1.PM5 can be viewed from different perspectives, namely, the site of action and the general nature of the drug-cell interaction. l. Killing foreign organisms. Chemotherapeutic agents act by killing or weakening foreign organisms such as bacteria, worms, and viruses. The main principle of action is selec￾tive toxicity, i.e. the drug must be more toxic to the parasite than to the host. 2. Stimulation and depression. Drugs act by stimulating or depressing normal physi￾ological functions. Stimulation increases the rate of activity while depression reduces it. 3. Irritation. It is a non-specific action of a drug that can occur in all the body tissues. Certain drugs act by causing irritation. Ex: Drugs like senna and castor oil show their laxative effects by their irritant action on gastrointestinal tract. 4. Replacement. Drugs serve as replacement of essential body chemicals that are ei￾ther absent or present in less than required quantity due to disease. Ex: Insulin is used in diabetes. Levodopa therapy in Parkinson’s disease.  
  A drug act by virtue of its various properties like physical, chemical, physiological etc. The fundamental mechanisms of drug action can be distinguished into following categories. 1. Physical Properties A physical property of the drug is responsible for its action. (i) Taste. Bitter taste drugs increase the flow the hydrochloric acid reflexly in the stom￾ach. Ex: Quassia, Chirata (ii) Mass. By increasing the bulk of drug in intestine produce laxative effect. Ex: Isapgol (iii) Adsorption. Certain drugs like kaolin adsorb water on to its surface and there by reduce gastric motility (iv) Radioactivity. The radioactive substances are commonly used to treat cancer. Ex: 125. 2. Chemical Properties The drugs react extracellularly according to simple chemical reactions like neutraliza￾tion, chelation, oxidation etc. Ex: (i) Aluminium hydroxide neutralizes acid in stomach (ii) Toxic heavy metals can be eliminated by chelating agents like EDTA, BAL, penicillamine etc. (iii) Oxidising agents are germicidal. 3. Through Enzymes Enzymes are very important targets of drug action because almost all biological reac￾tions are carried out under the influence of enzymes. Drugs may either increase or decrease enzymatic reactions. Ex: (i) Adrenaline stimulates adenyl cyclase (ii) Pyridoxine acts as a cofactor and increases decarboxylase activity