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Pharmacokinetics Made Simple

How Long Does Your Medication
Stay In Your System?

Enter your dose and see a personalized clearance timeline for 200+ prescription drugs — instantly, no account needed.

Half-life values sourced from FDA prescribing information & published pharmacokinetic studies
SELECT A MEDICATION *
🔍
Selected
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Half-Life
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Full Clearance (~5×t½)
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Category
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Enter the amount in milligrams you took
How many hours ago did you take the medication?
ℹ️
For reference only — not medical advice. Results show population averages and vary by individual factors (age, kidney/liver function, body weight). Always follow your doctor's or pharmacist's guidance. In emergencies, call 911.
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Remaining in System Now
—
— of original dose
Estimated Full Clearance
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from time of dose
⏱ Time Left to Clear
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from right now
Half-Life Used
—
typical adult value
Clearance Timeline Chart
Drug concentration in plasma over time · Dashed line = current moment · Based on first-order elimination kinetics
Note on drug interactions: When two medications share the same metabolic pathway, one can slow the other's clearance to dangerous levels — or speed it up until the drug barely works — because the enzyme handling one drug is either blocked or turbo-charged by the other. Read blog article

When taking more than one medication, the information on this page and the results shown in the calculator might not be correct. Always consult a qualified healthcare professional.
Detailed Timeline
Time Since Dose Amount Remaining % of Dose Level Status

How Long Do Medications Stay In Your System?

STEP 01

Select your medication

Choose from 200+ drugs across all major categories. Each has a pre-filled half-life from FDA prescribing information. Popular: Sertraline, Metformin, Fluoxetine, Ibuprofen

STEP 02

Enter your dose & timing

Input how many milligrams you took and how many hours ago. Both fields are required to calculate your personal clearance curve.

STEP 03

See your drug clearance curve

The calculator applies first-order elimination kinetics — the same model used in clinical pharmacokinetics — to show exactly how much drug remains at each timepoint.

NOTE

Individual variation matters

Age, kidney/liver function, genetics (CYP450), and drug interactions all affect real-world clearance. This tool shows the population average for a healthy adult.

Drug Half-Life Calculator — Frequently Asked Questions

How long does your medication stay in your system? Answers to the most common questions about drug clearance, half-life, and how medications are eliminated from the body.

What is drug half-life?+
Drug half-life (t½) is the time it takes for the concentration of a drug in your bloodstream to decrease by 50%. Most drugs require approximately 5 half-lives to be considered fully cleared from your system (below 3% of the original dose). Multiply the half-life by 5 to get the approximate full clearance time.
How long does medication stay in your system?+
It depends entirely on the drug. Short-acting medications like ibuprofen clear in 10–20 hours. Long-acting drugs like fluoxetine (Prozac) can take 4–6 weeks. Select any drug in the calculator above to see its specific clearance timeline.
Why does clearance vary between people?+
Liver and kidney function, age, genetics (CYP450 enzyme variants), body weight, and other medications all affect how quickly your body eliminates a drug. This calculator shows the population average for a healthy adult — individual results will vary.
Is this calculator accurate for drug testing purposes?+
No. Drug tests detect metabolites using different methods with different detection windows. Cannabis, for example, can be detected in urine for weeks despite clearing the blood quickly. Do not use this calculator to predict drug test results.
What is the half-life formula and how do you calculate it?+

The half-life of a drug is derived from first-order elimination kinetics, where a constant fraction of the drug is removed per unit time. The core formula is:

t½ = ln(2) / k ≈ 0.693 / k

where k is the elimination rate constant (per hour) and ln(2) ≈ 0.693.

1. Calculating the rate constant k from two blood samples:

k = ln(C₀ / C) / t

Measure the concentration at time 0 (C₀) and again at time t (C). Plug into the formula to get k, then divide 0.693 by k to get the half-life.

2. Calculating remaining drug amount after time t:

C = C₀ × (1/2)^(t / t½) or equivalently C = C₀ × e^(−kt)

Worked example: A 100 mg dose of a drug with a 4-hour half-life. After 8 hours (2 half-lives), the remaining amount is 100 × (1/2)² = 25 mg. After 12 hours (3 half-lives) → 12.5 mg. After 20 hours (5 half-lives) → ~3 mg (about 3% — the threshold clinicians use to call a drug "fully cleared").

The "5 half-lives" rule: After each successive half-life the remaining fraction is 50%, 25%, 12.5%, 6.25%, 3.125%. That is why every drug page on this site multiplies the published half-life by 5 to estimate full clearance time.

Note: This formula assumes single-compartment, first-order kinetics. Some drugs (alcohol, phenytoin at high doses, aspirin in overdose) follow zero-order kinetics where a constant amount — not fraction — is cleared per unit time, and the half-life formula above does not apply.