1. What is Drug Half-Life?

The half-life (t_1/2) of a drug is the time required for the concentration of the drug in the body to be reduced by half. It's a key pharmacokinetic parameter that helps determine dosing intervals and duration of drug action.

2. How Does the Calculator Work?

The calculator uses the half-life equation:

Where:

• \( t_{1/2} \) — Drug half-life (hours)
• \( k_{el} \) — Elimination rate constant (1/hour)
• 0.693 — Natural logarithm of 2 (ln 2)

Explanation: The equation shows the inverse relationship between elimination rate constant and half-life. Drugs with faster elimination (higher k_el) have shorter half-lives.

3. Importance of Half-Life Calculation

Details: Knowing a drug's half-life helps determine dosing frequency, time to reach steady state, and time for drug elimination from the body. It's crucial for therapeutic drug monitoring and avoiding toxicity.

4. Using the Calculator

Tips: Enter the elimination rate constant (k_el) in 1/hour units. The value must be greater than zero. The calculator will compute the corresponding half-life in hours.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical drug half-life range?
A: Half-lives vary widely from minutes (e.g., adenosine) to weeks (e.g., amiodarone). Most drugs have half-lives between 1-24 hours.

Q2: How many half-lives to eliminate a drug?
A: About 5 half-lives for 97% elimination. This is also the time needed to reach steady state with regular dosing.

Q3: Does half-life change with dose?
A: For first-order kinetics (most drugs), half-life is constant. For zero-order kinetics (e.g., phenytoin at high doses), half-life increases with dose.

Q4: What factors affect drug half-life?
A: Metabolism, excretion, protein binding, volume of distribution, and patient factors like age, liver/kidney function.

Q5: How is k_el determined?
A: Typically calculated from the slope of the terminal phase of the drug concentration-time curve on a semi-log plot.