Half life

Definition(Plasma half life):The time by which a measure will come to half is called the half of that measure.Which the measure may be conc of a drug that is called plasma half life(t ½) or may be the effect of a drug that is called biological effect half life or may be the excertion of a drug that called elimination half life.

Types of half life:

• 1)      Plasma t ½:The time by which conc of a drug will come to half is called the half of conc of a drug.
• 2)      Biological t ½ :The time by which the effect of a drug will come to half is called the biological half life of that drug.
• 3)      Elimination t ½:The time by which the excretion of a drug will come to half is called elimination half life of that drug.

Factors that can affecting half life:

• 1)      Route of administration.(oral/subcutaneous=More plasma t ½ )
• 2)      Plasma protien binding of a drug.(↑ PP binding=t ½ ↑)
• 3)      Tissue binding of a drug.(↑ tissue binding=t ½ ↑)
• 4)      Metabolism of a drug.(↑ Metabolism=t ½ ↓)
• 5)      Excretion of a drug.(↑ Excretion=t ½ ↓)
• 6)      Pathology in kidney,liver.(t ½ ↑)

Importance of half life:

• 1)      Half life gives a gross idea about the pharmacokinetics & pharmacodynamics of a drug.
• 2)      Half life gives idea about the duartion of action of a drug.
• 3)      It can guide the dosage schedules:

·         Low t ½ =Frequent administration.

·         High t ½ =Should be given once or twice daily.