Drug Half Life CalculatorHow Long Does a Medication Stay in Your System?
Estimate how much of a dose is still in your body, when it will be about 97% cleared, how long a regular medicine takes to reach steady state, and calculate half-life from two blood levels — with live charts and 30 common drug presets.
Uses standard first-order (exponential) elimination equations taught in pharmacy school and described by the NIH StatPearls review on half-life. Drug values reflect typical adult figures from FDA labeling on DailyMed.
Written & reviewed by Manzoor Ahmad, Pharm-DPharmacist · 9+ years writing on medicines & supplements · Updated Oct 18, 2026
Your dose
Useful for “when is it below a level I care about?” questions.
Your result
About 50 mg of your 400 mg dose is still in your body.
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Elimination curve
Clearance timeline
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Half-life ladder for this dose
| Half-lives | Time after dose | Amount left | % eliminated |
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Pharmacist insights
Your regimen
Once daily = 24 h, twice daily = 12 h, weekly = 1 week.
Steady state
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Build-up to steady state
Dose-by-dose build-up
| Dose # | Time | Peak after dose | Trough before next | % of steady state |
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Pharmacist insights
Two measured levels
Both samples should be taken in the elimination phase — after absorption and distribution are complete.
Calculated half-life
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Fitted decay curve
Pharmacist insights
What is a drug’s half-life?
A drug’s half-life (t½) is the time it takes for the amount of the drug in your bloodstream to fall by 50%. Most medicines follow first-order elimination, so the same fraction leaves every half-life: 50% remains after one, 25% after two, 12.5% after three, and only about 3% after five half-lives — the point at which a drug is considered essentially cleared from your system.
That single number drives almost every practical question I get at the pharmacy counter: When does this wear off? Can I take my next dose early? When is it safe to switch to another medicine? Why do I only feel the full effect after a week? A drug half life calculator turns the textbook rule into real hours and days for your exact dose.
Half-life is a property of how your body handles the drug, not of the dose. Doubling the dose does not double the time it lasts — under first-order kinetics it adds roughly one extra half-life before levels fall to the same point. That is why “taking more so it lasts longer” is a poor strategy and a common cause of side effects.
How to use this medication half-life calculator
- Pick your mode. “Single dose” shows how much is left after one dose; “Repeat dosing” shows build-up and steady state for a regular medicine; “Find half-life” works it out from two lab levels.
- Choose a drug preset or enter your own half-life. Presets load a typical adult value. If your prescriber or the package insert gives a different figure, type it in — the selector switches to “Custom” automatically.
- Enter the dose and time since you took it. Use any unit (mg, mcg, mL, IU). Results come back in the same unit.
- Read the verdict, gauge and chart. The clearance timeline shows the clock time when you reach 50%, 75%, 90%, 97% and 99% eliminated.
- Copy or reset. “Copy” puts a plain-text summary on your clipboard to share with your doctor or pharmacist.
Drug half-life formulas used
Amount remaining after time t
A(t) = Dose × (½)^(t ÷ t½) = Dose × e^(−kₑ·t)
Elimination rate constant
kₑ = ln 2 ÷ t½ = 0.693 ÷ t½
Time to fall to a target amount
t = t½ × log₂(Dose ÷ Target)
Half-life from two levels
t½ = 0.693 × Δt ÷ ln(C₁ ÷ C₂)
Repeat dosing (every τ hours)
Accumulation R = 1 ÷ (1 − e^(−kₑ·τ)) · Peakₛₛ = Dose × R · Troughₛₛ = Peakₛₛ × e^(−kₑ·τ) · Averageₛₛ = Dose ÷ (kₑ·τ)
Time to steady state depends only on half-life: about 90% after 3.3 half-lives and 97% after 5, whatever the dose or interval.
The model is a one-compartment, first-order system with instant absorption. That makes it ideal for understanding and estimating, and it matches how pharmacists do quick bedside maths — but it is not a substitute for therapeutic drug monitoring, which uses measured levels and drug-specific models.
The 5 half-lives rule: how much drug is left
This half-life elimination chart applies to every first-order drug, from ibuprofen to sertraline — only the clock time changes.
| Half-lives | % remaining | % eliminated | What it means |
|---|---|---|---|
| 1 | 50% | 50% | Half the dose is gone; effects usually still present |
| 2 | 25% | 75% | Short-acting drugs often wearing off |
| 3 | 12.5% | 87.5% | Most drugs below the effective level |
| 3.32 | 10% | 90% | 90% cleared; 90% of steady state when dosing regularly |
| 4 | 6.25% | 93.75% | Approaching washout |
| 5 | 3.125% | 96.9% | Clinically “cleared” — the standard washout rule |
| 6.64 | 1% | 99% | Practically all of the parent drug gone |
| 7 | 0.78% | 99.2% | Used for extra-cautious washouts |
Half-life of common medications and supplements
Typical healthy-adult elimination half-lives from product labeling and standard references. Individual values vary widely — the range column shows how much. These are the same values loaded by the calculator presets.
| Drug | Typical t½ | Usual range | ~97% cleared (5 × t½) |
|---|---|---|---|
| Melatonin (immediate-release) | 45 min | 30–60 min | ≈ 4 h |
| Amoxicillin | 1 h | 1–1.3 h | ≈ 5 h |
| Omeprazole | 1 h | 0.5–1 h (acid effect lasts far longer) | ≈ 5 h |
| Ibuprofen | 2 h | 1.8–2.5 h | ≈ 10 h |
| Nicotine | 2 h | 1–3 h (cotinine ≈ 16 h) | ≈ 10 h |
| Acetaminophen (paracetamol) | 2.5 h | 2–3 h | ≈ 12.5 h |
| Zolpidem | 2.5 h | 1.4–4.5 h | ≈ 12.5 h |
| Prednisone | 3 h | 2–4 h (effects 12–36 h) | ≈ 15 h |
| Caffeine | 5 h | 3–7 h | ≈ 25 h |
| Gabapentin | 6 h | 5–7 h | ≈ 30 h |
| Metformin | 6.2 h | 4–9 h (plasma) | ≈ 31 h |
| Tramadol | 6.3 h | 5.5–7.5 h (M1 metabolite ≈ 7.4 h) | ≈ 32 h |
| Diphenhydramine | 8 h | 2.4–9.3 h (longer in older adults) | ≈ 40 h |
| Cetirizine | 8.3 h | 6.5–10 h | ≈ 42 h |
| Loratadine | 8.4 h | 3–20 h (desloratadine ≈ 28 h) | ≈ 42 h |
| Alprazolam | 11 h | 6–27 h | ≈ 2.3 days |
| Lorazepam | 12 h | 10–20 h | ≈ 2.5 days |
| Lisinopril | 12 h | effective t½ for accumulation | ≈ 2.5 days |
| Atorvastatin | 14 h | active metabolites 20–30 h | ≈ 3 days |
| Lithium | 24 h | 18–36 h (longer in kidney impairment) | ≈ 5 days |
| Sertraline | 26 h | 22–36 h | ≈ 5.4 days |
| Escitalopram | 30 h | 27–32 h | ≈ 6.3 days |
| Warfarin | 40 h | 20–60 h | ≈ 8.3 days |
| Amlodipine | 40 h | 30–50 h | ≈ 8.3 days |
| Digoxin | 40 h | 36–48 h (up to 5 days in renal failure) | ≈ 8.3 days |
| Diazepam | 43 h | 20–50 h (desmethyldiazepam up to 100 h) | ≈ 9 days |
| Azithromycin | 68 h | tissue half-life is long | ≈ 14 days |
| Fluoxetine | 4 days | 1–3 days acute, 4–6 chronic (norfluoxetine 4–16 days) | ≈ 3 weeks |
| Tirzepatide | 5 days | ≈ 5 days | ≈ 25 days |
| Semaglutide (weekly injection) | 7 days | ≈ 1 week | ≈ 5 weeks |
| Levothyroxine | 7 days | 6–7 days (3–4 in hyperthyroidism, 9–10 in hypothyroidism) | ≈ 5 weeks |
| Vitamin D — 25(OH)D | 15 days | 2–3 weeks | ≈ 11 weeks |
Values are population averages for healthy adults and are rounded. Always use the figure on your product’s label or from your clinician when it differs.
Worked example: how long does ibuprofen stay in your system?
This is the calculator’s default setting. You take 400 mg of ibuprofen, which has a half-life of about 2 hours, and want to know what’s left 6 hours later.
- Count the half-lives: 6 h ÷ 2 h = 3 half-lives.
- Apply the fraction: (½)³ = 0.125, so 400 mg × 0.125 = 50 mg remaining (12.5%), meaning 87.5% has been eliminated.
- Find the washout time: 5 × 2 h = 10 hours to reach about 97% cleared, and 6.64 × 2 h ≈ 13.3 hours for 99%.
- Repeat dosing: 400 mg every 6 h gives an accumulation factor of 1.14, a steady-state peak of about 457 mg and a trough of about 57 mg — reached in roughly 10 hours.
Notice that by the 6-hour mark — the usual redosing interval — ibuprofen’s pain relief has largely faded because most of the dose is gone. Its anti-inflammatory effect at the tissue level outlasts blood levels slightly, which is one reason half-life and “duration of action” are not identical.
What changes a drug’s half-life in real life?
Half-life depends on two things: how widely a drug spreads into body tissues (volume of distribution) and how fast your liver and kidneys clear it (clearance). Expressed as a formula, t½ = 0.693 × Vd ÷ CL. Anything that changes either side changes the half-life.
Kidney function
Drugs cleared unchanged by the kidneys — gabapentin, metformin, lithium, digoxin, many antibiotics — can have half-lives two to five times longer when kidney function is reduced. Your prescriber estimates this with creatinine clearance before choosing a dose.
Liver function and enzymes
Liver disease slows drugs broken down by CYP450 enzymes, such as diazepam and many antidepressants. Interacting drugs can inhibit these enzymes (lengthening t½) or induce them (shortening it). Smoking induces CYP1A2, roughly halving caffeine’s half-life.
Age
Older adults often have lower kidney function and more body fat, so fat-soluble drugs like diazepam and diphenhydramine linger longer. Newborns clear many drugs very slowly — caffeine’s half-life in a newborn can exceed 80 hours.
Pregnancy, body weight and genetics
Pregnancy speeds the clearance of some drugs and slows others (caffeine roughly doubles to triples in late pregnancy). Obesity increases distribution of fat-soluble drugs. Genetic “poor metabolizer” status for CYP2D6 or CYP2C19 can stretch half-lives considerably.
Half-life vs. how long a drug works vs. drug-test windows
People often treat these three as the same thing. They aren’t, and mixing them up leads to wrong decisions.
- Half-life measures how fast the parent drug leaves your blood.
- Duration of action is how long the effect lasts. Omeprazole’s half-life is about an hour, yet it blocks acid pumps for a day or more. Aspirin’s antiplatelet effect lasts the life of the platelet — around a week — despite a very short half-life.
- Detection window is how long a test can find the drug or its metabolites in urine, blood, saliva or hair. Labs often look for long-lived metabolites, and urine concentrates them, so detection can run far beyond five half-lives of the parent drug.
Why regular medicines take days to “kick in”: steady state
When you take a medicine on a schedule, each dose lands on top of what’s left from the last one. Levels climb until the amount eliminated per interval equals the dose taken — that plateau is steady state. It takes about 4–5 half-lives no matter how big the dose is or how often you take it.
That explains a lot of real-world experiences. Sertraline (t½ ≈ 26 h) takes about five to six days to reach a stable level. Levothyroxine (t½ ≈ 7 days) takes five to six weeks, which is why thyroid blood tests are rechecked six weeks after a dose change. For drugs where waiting is not practical, clinicians may use a loading dose — the repeat-dosing tab estimates it as the maintenance dose × the accumulation factor.
Notes from the pharmacy counter
After nine-plus years of counseling patients and writing about medicines and supplements, the half-life question I hear most is about switching. Someone stopping fluoxetine to start another antidepressant, for example, needs a far longer gap than with sertraline, because fluoxetine’s active metabolite has a half-life measured in weeks. A five-half-life washout of norfluoxetine can stretch past a month. Never time a switch yourself — your prescriber will plan it.
On the supplement side, half-life explains why melatonin (t½ under an hour) rarely causes a next-day hangover at sensible doses, while a high-dose diphenhydramine sleep aid often does. It is also why vitamin D blood levels respond slowly: with a 25(OH)D half-life of two to three weeks, a dosing change takes two to three months to show its full effect on a lab test.
Finally, a missed dose of a long half-life drug is usually far less dramatic than people fear, while a missed dose of a very short-acting one can matter. Check your leaflet or ask your pharmacist rather than doubling up.
Common mistakes when calculating drug half-life
- Ignoring active metabolites. Diazepam, fluoxetine, tramadol and loratadine all produce metabolites that keep working after the parent drug is gone.
- Using half-life for zero-order drugs. Alcohol and high-dose phenytoin are eliminated at a roughly constant amount per hour, so the half-life concept does not apply in the usual way.
- Treating extended-release products like immediate-release. ER tablets keep releasing drug for hours, so levels fall later than this simple model predicts.
- Assuming “cleared” means “no effect.” Some drugs act irreversibly (aspirin, proton pump inhibitors) or change receptors over weeks (antidepressants).
- Taking an extra dose to make it last longer. Doubling the dose only buys about one more half-life and raises the peak — the main driver of side effects.
Drug half-life FAQs
Multiply the drug’s elimination half-life by 5. After five half-lives about 97% of the drug has been eliminated, which is the standard point at which it is considered cleared. For a drug with a 6-hour half-life, that is roughly 30 hours.
About 4 to 5 half-lives. After 4 half-lives roughly 94% is gone and after 5 about 97%. For 99% elimination you need about 6.6 half-lives.
The amount left is Dose × (½)^(time ÷ half-life). Half-life itself equals 0.693 ÷ kₑ, or 0.693 × volume of distribution ÷ clearance. From two blood levels, t½ = 0.693 × Δt ÷ ln(C₁ ÷ C₂).
Only slightly. For most drugs the half-life stays the same at any dose, so doubling the dose adds about one extra half-life before levels fall to the same point. A higher dose mainly raises the peak level and the risk of side effects.
About 4 to 5 half-lives of regular dosing, regardless of dose or how often you take it. A drug with a 24-hour half-life reaches steady state in about 4–5 days; one with a 7-day half-life takes 4–5 weeks.
No. Half-life describes blood levels, while duration of action describes the effect. Omeprazole and aspirin have short half-lives but long-lasting effects, while some drugs stop working well before they are cleared.
No. Drug tests often detect metabolites that last longer than the parent drug, and urine, hair and saliva tests have their own detection windows and cut-off levels. Half-life gives only a rough lower bound on how long a substance may be found.
Half-life varies between people because of age, kidney and liver function, body composition, genetics, pregnancy, smoking and interacting medicines. Published values are averages from clinical studies, so ranges show the realistic spread.
Drugs removed mainly by the kidneys, such as gabapentin, lithium, digoxin and metformin, can have half-lives several times longer when kidney function is reduced. Doses are often lowered or spaced out based on creatinine clearance.
kₑ is the fraction of drug removed per unit of time in first-order elimination. It equals 0.693 divided by the half-life, so a drug with a 2-hour half-life has a kₑ of about 0.347 per hour, meaning roughly 29% of what is present leaves each hour.
Not in the usual way. Alcohol follows zero-order kinetics at typical drinking levels, meaning the liver removes a roughly fixed amount per hour regardless of how much is in the blood. A half-life calculator is not suitable for estimating blood alcohol.
Use it to understand your medicine, not to change how you take it. Always follow the dosing schedule on your prescription or label, and ask your pharmacist or doctor before taking doses early, late, or switching medicines.
Manzoor Ahmad, Pharm-D
Pharmacist · Medicines & supplements writer · 13+ years in web development & SEOManzoor is a practicing pharmacist with over nine years of experience writing about medicines, supplements and dosing. He builds Dexocalc’s health calculators using the same pharmacokinetic equations used in pharmacy practice, and reviews every tool against FDA labeling and peer-reviewed references.
Sources and references
- Hallare J, Gerriets V. Half Life. StatPearls, NCBI Bookshelf, National Library of Medicine.
- Grogan S, Preuss CV. Pharmacokinetics. StatPearls, NCBI Bookshelf.
- U.S. National Library of Medicine. DailyMed — FDA-approved drug labeling (clinical pharmacology sections for individual half-life values).
- Certara. Understanding steady-state pharmacokinetics.
- U.S. Food and Drug Administration. Drug development and drug interactions: substrates, inhibitors and inducers.
Medical disclaimer: This drug half life calculator provides educational estimates based on a simplified one-compartment, first-order model and typical adult values. It does not account for extended-release formulations, absorption time, active metabolites, organ impairment, drug interactions or individual variability. It is not medical advice and is not a substitute for your doctor or pharmacist. Do not start, stop or change any medication based on these results. Not suitable for estimating blood alcohol or drug-test outcomes.
Last reviewed: October 18, 2026 · Dexocalc Health Calculators