Half-life of drug is the length of time required for the concentration of a particular drug substance to decrease to half of its starting dose in the body. Half-life of a drug can be calculated by following equations:
Half-life Calculation Formula
Equation 1:
t1/2= 0.693/k
Equation 2:
t1/2= (0.693×Vd)/CL
Where,
- Vd = the volume of distribution
- CL = clearance
Note: The volume of distribution (Vd) is the ratio of the amount of drug in the body to the drug concentration in the plasma (amount of drug in body/plasma drug concentration).
For example,
If 50 mg of a drug with a half-life of 60 minutes is taken, the following is estimated:
- 60 minutes after administration, 25mg remains
- 120 minutes after administration, 12.5mg remains
- 180 minutes after administration, 6.25mg remains
- 240 minutes after administration, 3.125mg remains
Factors that may Affect Half-life of a Drug
- Age
- Gender
- Blood circulation
- Kidney function
- Drug formulation
- Volume of distribution
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