How to Calculate Half-life of a Drug?

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 


  • 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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