Calculate drug elimination, time to clearance, steady-state concentrations, and accumulation factor from half-life and dosing interval.
The half-life of a drug is the time required for the plasma concentration — or the total amount of drug in the body — to decrease by exactly 50%. This single pharmacokinetic parameter governs how quickly a drug is eliminated, how long it takes to reach steady state with repeated dosing, and when a drug is effectively cleared from the body. Understanding half-life is essential for determining dosing intervals, predicting drug interactions, planning washout periods before surgery, and interpreting therapeutic drug monitoring results.
This drug half-life calculator computes the amount of drug remaining in the body after any given time period, the time required to eliminate a target percentage of the dose, the time to reach steady-state concentrations with repeated dosing, and the accumulation factor that determines peak and trough levels at steady state. It includes presets for commonly prescribed medications — from short-acting drugs like ibuprofen (2 hours) to long-acting drugs like fluoxetine (72 hours) — and generates a visual decay table showing how drug levels fall over successive half-lives.
For drugs following first-order kinetics (the vast majority), the elimination rate is proportional to the drug concentration. This means a constant fraction of the drug is eliminated per unit time, and the half-life remains constant regardless of dose size. The practical rule of thumb is that a drug is approximately 97% eliminated after 5 half-lives, and steady-state is achieved after 4–5 half-lives of repeated dosing at the same interval.
Understanding drug half-life is critical for safe prescribing but the math can be non-trivial. This calculator instantly determines how much drug remains at any time point, when a drug will be effectively cleared, and what accumulation to expect with repeated dosing — essential for dose adjustments, washout calculations, and therapeutic drug monitoring interpretation.
Amount remaining = Dose × (0.5)^(time / half-life). Elimination constant: ke = ln(2) / half-life. Time to target elimination: t = -ln(1 - target%) / ke. Accumulation factor = 1 / (1 - e^(-ke × dosing interval)). Steady-state peak = Dose × accumulation factor. Steady-state trough = peak × e^(-ke × dosing interval).
Result: 62.5 mg remaining (12.5%) after 18 hours
A 500 mg dose with a 6-hour half-life: after 18 hours (3 half-lives), 500 × 0.5³ = 62.5 mg remains (12.5%). With an 8-hour dosing interval, the accumulation factor is 1.55, giving a steady-state peak of 775 mg and trough of 258 mg.
Most drugs follow first-order elimination kinetics, where a constant fraction (not a constant amount) is eliminated per unit time. This produces an exponential decay curve with a constant half-life. Zero-order kinetics occur when metabolic enzymes are saturated — as with ethanol and phenytoin at supratherapeutic levels — and a fixed amount is eliminated regardless of concentration. In zero-order cases, the concept of half-life does not strictly apply, and elimination time is linearly proportional to dose.
Half-life directly informs several clinical decisions: (1) Dosing interval selection — drugs with short half-lives need more frequent dosing; (2) Loading dose necessity — drugs with very long half-lives may need loading doses to achieve therapeutic levels quickly rather than waiting for 5 half-lives; (3) Washout planning — before surgery or switching medications, the washout period is 5 half-lives; (4) Therapeutic drug monitoring timing — levels should be drawn at steady state, typically after 4–5 half-lives of a consistent dose.
For drugs administered by continuous infusion, the "context-sensitive half-time" describes how long it takes for plasma concentration to fall by 50% after stopping an infusion. This differs from the elimination half-life because it depends on the duration of infusion and the drug's distribution properties. This concept is particularly important in anesthesiology, where drugs like propofol and remifentanil have very different context-sensitive half-times despite similar elimination profiles.
Half-life (t½) is the time needed for half the drug to be eliminated from the body. After 1 half-life, 50% remains; after 2 half-lives, 25%; after 3 half-lives, 12.5%, and so on. It determines dosing frequency and clearance time.
A drug is considered effectively eliminated after 5 half-lives, at which point approximately 97% has been cleared (only 3.125% remains). For most clinical purposes, 5 half-lives is the standard washout period.
Steady state occurs when the rate of drug intake equals the rate of elimination, resulting in consistent peak and trough levels. It takes 4–5 half-lives to reach steady state. This is when drug levels and effects become predictable and stable.
The accumulation factor quantifies how much drug accumulates with repeated dosing at steady state compared to a single dose. It depends on the dosing interval relative to the half-life. Shorter intervals or longer half-lives lead to greater accumulation.
For drugs following first-order kinetics (most drugs), the half-life is constant regardless of dose. However, some drugs (like ethanol and phenytoin at high doses) follow zero-order kinetics where a fixed amount — not percentage — is eliminated per unit time.
Half-life is affected by hepatic function, renal function, age, genetic polymorphisms (e.g., CYP enzymes), drug interactions, protein binding, body composition, and disease states. Liver or kidney impairment typically increases half-life and slows drug clearance.