# 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

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