How to calculate Drug Half-life?

Drug half-life is the time it takes for the concentration of a drug in the body to be reduced by half. This is a crucial pharmacokinetic parameter that helps determine dosing intervals and predict drug accumulation or elimination from the body.

The half-life (t½) can be calculated using the following formula, which is derived from the first-order elimination kinetics:

t½ = (t * ln(2)) / ln(C0 / Ct)

Using the Drug Half-life Calculator: an example

Let's consider a scenario where a patient is administered a drug, and we want to determine its half-life based on observed concentrations over time.

Step-by-step calculation:

• Initial Concentration (C0): 100 mg/L
• Current Concentration (Ct): 25 mg/L
• Time Elapsed (t): 4 hours

Using the formula:

t½ = (4 * ln(2)) / ln(100 / 25)

t½ = (4 * 0.693) / ln(4)

t½ = 2.772 / 1.386

t½ = 2 hours

What do the results mean?

A half-life of 2 hours means that every 2 hours, the concentration of the drug in the patient's body will decrease by 50%. This information is vital for healthcare professionals to determine how often a drug needs to be administered to maintain therapeutic levels, or how long it will take for the drug to be completely eliminated from the system.

Frequently Asked Questions

• Why is drug half-life important? It helps determine dosing frequency, predict drug accumulation, and understand withdrawal times.
• Does half-life vary among individuals? Yes, factors like age, kidney/liver function, and genetics can influence a drug's half-life.
• What is steady state? It's when the amount of drug administered is equal to the amount eliminated, leading to stable drug levels. It typically takes about 4-5 half-lives to reach steady state.
• Can half-life be affected by other drugs? Yes, drug interactions can either increase or decrease a drug's half-life.