Drug Half-Life
Time for the plasma concentration of a drug to fall to half its initial value.
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Example
kₑ = 0.1/hr → t½ = 6.93 hours
How it works
Drug half-life is the time it takes for a drug’s plasma concentration to drop by 50% when the body removes it by first-order elimination. The constant 0.693 is ln 2, which appears because first-order decay is exponential: after one half-life, the amount remaining is exactly half of what it was. Since the elimination rate constant ke tells you how quickly the drug is cleared per unit time, dividing ln 2 by ke gives the characteristic time for the concentration to halve.
When to use it
Use this formula when a drug follows first-order kinetics and you know the elimination rate constant from pharmacokinetic data. It is helpful for estimating dosing intervals, time to steady state, and how long a drug will stay in the body after dosing stops.
Worked example, step by step
Given: ke = 0.25 hr^-1 Step 1: Write the half-life formula: t1/2 = 0.693 / ke Step 2: Substitute the value: t1/2 = 0.693 / 0.25 Step 3: Calculate: t1/2 = 2.772 hours Final answer: The drug half-life is about 2.77 hours.
Common mistakes to avoid
- Using this formula for zero-order elimination, where half-life is not constant
- Mixing up the elimination rate constant ke with clearance or dose
- Forgetting that ke must have time^-1 units, otherwise the half-life calculation will be inconsistent
Did You Know?
After five half-lives, about 97 % of a drug has been eliminated — this '5 half-lives' rule guides how long drugs stay active.
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More in Pharmacology & Dosage
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IntermediateAverage plasma drug concentration when intake and elimination are balanced.
Loading Dose
IntermediateInitial higher dose to rapidly achieve the desired plasma concentration.
Maintenance Dose
IntermediateRecurring dose needed to sustain the steady-state concentration.
Volume of Distribution
BasicApparent volume into which a drug distributes — a larger Vd means the drug leaves the bloodstream and enters tissues.
Clearance
IntermediateVolume of plasma completely cleared of a drug per unit time.
Bioavailability
IntermediateFraction of an oral dose that reaches systemic circulation compared with an IV dose.