Drug Half-Life Calculator
Enter the initial dose and the half-life of a drug to find out how much remains in the body at any elapsed time, how long it takes to clear 50%, 95%, and 99% of the dose, and the full concentration-decay curve. Supports minutes, hours, and days. Use the reference table to look up the half-life of 25 common medications.
Formula
Worked example
A 500 mg dose of a drug with a 6-hour half-life: after 12 hours (2 half-lives), 500 x 0.5^2 = 125 mg remains (25%). After 24 hours (4 half-lives), 500 x 0.5^4 = 31.25 mg remains (6.25%). 95% clearance takes 6 x log2(20) = 25.9 hours. 99% clearance takes 6 x log2(100) = 39.9 hours.
What is drug half-life?
Drug half-life (t1/2) is the time it takes for the concentration of a drug in the body to fall to exactly half of its current value. It is a fundamental pharmacokinetic parameter that determines how often a drug needs to be taken and how long it stays in the system after the last dose. A drug with a 2-hour half-life clears much faster than one with a 40-hour half-life. Most drugs follow first-order (exponential) kinetics, meaning the same fraction is eliminated in each time period, not the same absolute amount.
How to use this calculator
Enter the initial dose in milligrams, the half-life in your preferred time unit (minutes, hours, or days), and the time elapsed since the dose was taken. The calculator instantly shows how many milligrams remain, what percentage of the original dose that represents, the number of half-lives elapsed, and how long it will take to reach 50%, 95%, and 99% elimination. The decay chart plots the full concentration-time curve, and the schedule table breaks it down half-life by half-life. Use the reference table to look up the half-life of 25 common medications. These results are educational only and not a substitute for medical or pharmaceutical advice.
The exponential decay formula
The standard pharmacokinetic model for first-order elimination is: C(t) = C0 x (0.5)^(t / t1/2), where C0 is the initial concentration (or dose), t is elapsed time, and t1/2 is the half-life. Because each half-life halves what remains, concentrations fall steeply at first and then more slowly. For example, after 1 half-life, 50% remains; after 2, 25%; after 3, 12.5%; after 4, 6.25%; after 5, about 3.1%. This is why clinicians often use the rule of thumb that 5 half-lives represent effective clearance, leaving only about 3% of the original dose.
Clearance thresholds: 50%, 95%, and 99%
Three clearance milestones are commonly used in clinical practice. The 50% point equals exactly one half-life and marks when half the dose has been eliminated. The 95% point occurs at approximately 4.32 half-lives (log2(20) x t1/2) and is often considered the practical threshold for drug interactions and washout periods. The 99% point occurs at approximately 6.64 half-lives (log2(100) x t1/2) and represents near-complete clearance relevant for stopping medications before surgery or starting a contraindicated drug. For amiodarone with its 26-107 day half-life, 99% clearance can take over a year.
Factors that change drug half-life
Half-life is not a fixed constant for every person. Kidney function has the biggest impact on renally cleared drugs: impaired kidneys slow elimination and effectively lengthen the half-life, causing drug accumulation. Liver disease similarly slows metabolism of hepatically processed drugs. Age matters because both renal and hepatic function typically decline with age. Body composition, genetic polymorphisms in drug-metabolizing enzymes (such as CYP2D6), drug-drug interactions that inhibit or induce metabolic enzymes, and pregnancy all shift the effective half-life from published averages. The values in the reference table are population averages for healthy adults.
Half-lives of common medications
| Drug | Typical half-life | Category |
|---|---|---|
| Epinephrine (Adrenaline) | 2-3 min | Very short |
| Lidocaine | 1-2 h | Short |
| Acetaminophen (Paracetamol) | 1-4 h | Short |
| Morphine | 1-7 h | Short |
| Propranolol | 2-6 h | Short |
| Oxycodone | 3-5 h | Short |
| Fentanyl | 3-12 h | Short |
| Heparin | 3-6 h | Short |
| Methotrexate | 3-10 h | Short |
| Ciprofloxacin | 4 h | Short |
| Streptomycin | 5-6 h | Short |
| Ganciclovir | 3-6 h | Short |
| Metronidazole | 8 h | Moderate |
| Tetracycline | 7-11 h | Moderate |
| Adderall (amphetamine) | 9-14 h | Moderate |
| Valproic Acid | 9-16 h | Moderate |
| Ketoconazole | 2-8 h | Moderate |
| Carbamazepine | 14-47 h | Long |
| Haloperidol | 14-41 h | Long |
| Diazepam | 21-37 h | Long |
| Fluconazole | 20-50 h | Long |
| Warfarin | 20-60 h | Long |
| Digoxin | 36-48 h | Long |
| Phenobarbital | 81-117 h | Very long |
| Amiodarone | 26-107 days | Very long |
Typical half-life ranges reported in clinical pharmacology literature. Individual values vary with age, kidney function, liver function, and drug interactions. Always consult a prescriber or pharmacist for patient-specific guidance.
Frequently asked questions
What does drug half-life tell me?
Half-life tells you how quickly a drug leaves the body. A short half-life means the drug clears fast and needs frequent dosing to maintain a therapeutic level; a long half-life means the drug stays in the system for days or weeks. Knowing the half-life helps predict how long a drug will be detectable in your body, how long side effects might last after stopping, and how long to wait before taking an interacting medication.
How many half-lives until a drug is completely out of my system?
In practice, a drug is considered effectively cleared after 5 half-lives (about 97% eliminated) or 7 half-lives (about 99% eliminated). True 100% clearance takes theoretically infinite time because the exponential model never quite reaches zero. For most clinical decisions, 5-7 half-lives is the accepted benchmark.
Does this calculator work for all drugs?
This calculator uses the standard first-order (exponential) decay model, which applies to the vast majority of drugs taken at normal therapeutic doses. Some drugs follow zero-order kinetics (a fixed amount is eliminated per hour, not a fixed fraction), which produces a linear rather than exponential decline. Ethanol (alcohol) is the most common example. Phenytoin and aspirin at high doses can also behave this way. Use this tool only for drugs known to follow first-order kinetics.
Can I use this calculator to find out when a drug will clear before a medical test?
This calculator can give you an estimate based on published average half-lives, but detection windows for drug testing depend on the specific test (blood, urine, hair, saliva), the sensitivity of the assay, your individual metabolism, hydration, and body fat content. Some metabolites are detectable long after the parent drug has cleared. For accurate information about a specific drug test, consult the testing facility or a healthcare provider.
What is the difference between half-life and duration of action?
Duration of action is how long a drug produces its therapeutic effect. Half-life is how long the drug takes to reduce to half its concentration. These are related but not the same. Some drugs have short half-lives but long durations of action because they bind tightly to receptors. Others have long half-lives but short effects because the pharmacological response desensitizes quickly. Always rely on clinical experience and prescribing information for duration of effect rather than calculating it from the half-life alone.
What is steady-state concentration?
When a drug is taken on a regular schedule, the amount eliminated between doses equals the amount added by each new dose. This balance is called steady state. It is typically reached after 4-5 half-lives of regular dosing. At steady state, the average concentration in the body stays constant from dose to dose. Drugs with very long half-lives, such as amiodarone, take weeks to months to reach steady state, which is why loading doses are sometimes used.