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Define half life and their types


Q: Define half life and their types.
Ans: Half life: The time required for a measure to decrease by one-half is called the half life of that measure.
Types of half life:
(i) Plasma half life (t112): The time required for the concentration of a drug to decrease by one half is called plasma half life.
(ii) Biological half-life (t112): The time required for the effects of a drug to decrease by one-half is called biological half life.
(iii) Elimination half-life (t112): The time required for the excretion of a drug to decrease by one half.
Q: What is the importance of half life? Ans: Importance of half life:
(i) Half life gives a gross idea about the pharmacokinetics and pharmacodynamics of a drug.
(ii) Half life gives idea about the duration of action of a drug.
(iii) It guides the dosage schedules. e.g.
Low t, Frequent administration
/2
e High t --> should be given once or twice daily. /2
a
Q: Define bio-transformation/metabolism. What are the objectives of bio-transformation?

Ans: Biotransformation:
These are the chemical reactions occuring within the body which reduces the lipid solubility and biological activity of drugs, thereby making them progressively more water soluble and favours for elimination (through kidney).
Objectives:
(i) Decreasing the lipid solubility.
(ii) Increasing the water solubility.
(iii) Conversion of unionized drug to ionized form.
(iv) Inactive substance (prodrug) is converted into active metabolite. Example: Talampicillin—> Ampicillin
Enalapril Enalaprilat
(v) More toxic drug is converted into less toxic drug. Example: Phenacetin ---> Acetaminophen.
Q: Why drug metabolism is necessary?
Ans: Drugs are the chemical substances and interact with living organisms. As we need drugs for treatment of disease similarly we also want to get rid of them from the body to overcome their persistent effects. 100mg of phenobarbitone would have a half life of

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