Medication Half-Life & Steady-State Planner
See rough steady-state and washout timing
Medication half-life, steady-state and washout FAQ
What does this half-life and steady-state planner do?
It takes an estimated elimination half-life for a medication and a usual dosing interval (for example, every 24 hours), then shows rough timing for when you are close to steady-state on treatment and when a drug is largely cleared after stopping, using standard half-life rules-of-thumb.
Is this a tool for choosing or changing medication doses?
No. It is strictly an education helper. It is not a dosing calculator, not a prescribing tool and not a way to decide how or when to start, stop, taper or adjust any medication. Those decisions always belong to your prescriber and care team.
What is “half-life” in this context?
Elimination half-life is the time it takes for the amount or concentration of a drug in the body to fall by 50% once it is distributed. After each half-life, about half of what is left is cleared again, so the amount falls in a stepwise-but-smooth curve rather than all at once.
Why do people say steady-state and washout both take 4–5 half-lives?
With repeated dosing or a constant infusion, each new dose adds drug while previous doses are being cleared. After 1 half-life you are at about 50% of the eventual steady-state level, after 2 half-lives about 75%, after 3 about 87.5% and so on. By around 4–5 half-lives you are close enough to steady-state (or, after stopping, have cleared most of the drug) for many practical decisions.
Where should I get the half-life value I enter here?
You should use the half-life quoted in a reliable source: the official patient information leaflet, prescribing information, or advice from your pharmacist, doctor or specialist nurse. Do not guess or assume a half-life from social media posts or similar.
Does this planner know which drug I am taking?
No. The calculator never identifies any specific drug and never stores entries as medical records. It only works with the generic numbers you type in and cannot substitute for personalised pharmacokinetics work done by clinicians or clinical pharmacists.
Who should be especially careful with half-life and timing?
People with kidney or liver disease, pregnancy, complex polypharmacy, organ transplants, advanced age, or narrow-therapeutic-index medicines (for example, some heart, seizure or transplant drugs) need close professional monitoring. For them, timing decisions must be based on formal advice and blood tests, not generic calculators.
How to use this medication half-life and steady-state planner
This page turns a half-life estimate and dosing interval into a simple, readable picture of how long it usually takes to get close to steady-state while you stay on a medication, and how long it can take until most of the drug is gone after you stop it. It is meant as a teaching aid to help you talk with your prescriber, not as a tool for changing anything on your own.
1. Choose half-life units and enter the value
Start with the units box. By default the calculator uses hours, which is how half-life is listed in many pharmacology sources. If your leaflet quotes a half-life in hours (for example, “t½ ≈ 12 hours”), leave the units as hours and type that number. If it is quoted in days, switch the box to days and enter, for example, 1.5 days. The calculator converts everything to hours in the background.
2. Add the usual dosing interval in hours
Next, enter how often the medicine is normally taken: every 8 hours, every 12 hours, every 24 hours, and so on. For once-daily medicines, 24 hours is a typical entry. For something taken every second day you could use 48 hours. This lets the tool estimate roughly how many doses it might take to approach steady-state.
3. Pick what you want to focus on
Use the Focus box to choose whether you mainly care about steady-state while on treatment, about washout after stopping, or about both. The calculations in the background are similar, but the wording in the summary changes so it is easier to apply to your actual questions.
4. Optionally add a dose and a day horizon
You can add a single typical dose in milligrams if you want the summary to talk about a “relative amount” in the body near steady-state. This is a simplified educational number, not a true blood level. You can also add a rough number of days (for example, 14) to see how the half-life landmarks relate to that overall time frame.
5. Read the steady-state and washout timing
When you hit Estimate steady-state and washout, the summary shows:
- About how long it takes to reach ~90–97% of steady-state, expressed in hours and in days.
- About how long after stopping it takes for ~90–97% of the drug to be cleared for the same half-life.
- A rough number of routine doses until you are near steady-state at your chosen interval.
- A short sentence you can copy into notes or messages when you talk with your prescriber.
6. Use the copy button for questions and check-ins
The Copy summary button creates a compact text summary that you can paste into a secure message, a clinic portal or your own notes. This can make it easier to ask questions like “Am I already at steady-state?” or “Roughly how long until this is mostly out of my system?” during appointments.
7. Treat everything here as a conversation starter
Half-life rules-of-thumb are approximations. Real-world pharmacokinetics depend on your organs, other medicines, genetics, formulation, food, illness and more. If the numbers here do not match what your prescriber says, always follow the plan you agreed together and use this tool to help you understand the logic, not to override it.
Never change, pause or combine medications without checking with your prescriber or pharmacist, even if a half-life explanation makes something look “simple” on paper.
How the half-life, steady-state and washout math works
The planner uses the classic one-compartment, first-order elimination model and very standard half-life rules-of-thumb. It keeps the numbers simple on purpose so you can link them to graphs or explanations in leaflets and textbooks.
1. Converting everything into hours
Internally the tool converts the half-life into hours:
- If you enter hours, it uses that value directly.
- If you enter days, it multiplies by 24 to convert to hours.
The dosing interval is also in hours, so the calculator can talk about how many half-lives pass across a certain number of doses.
2. Percent of steady-state reached over half-lives
For many regularly dosed drugs with first-order elimination, the fraction of steady-state reached depends mainly on how many half-lives have passed, not the exact dose:
- After 1 half-life: about 50% of steady-state.
- After 2 half-lives: about 75% of steady-state.
- After 3 half-lives: about 87.5% of steady-state.
- After 4 half-lives: about 94% of steady-state.
- After 5 half-lives: about 97% of steady-state.
The tool uses these landmarks to label “roughly at steady-state” rather than pretending there is a sharp on/off switch.
3. Estimating time to near steady-state and near washout
Once the half-life in hours is known, the calculator multiplies by a small whole number of half-lives:
Time (hours) = number of half-lives × half-life (hours)
For example, if the half-life is 24 hours, then:
- 3 half-lives ≈ 72 hours (about 3 days) for ~87.5% of steady-state.
- 5 half-lives ≈ 120 hours (about 5 days) for ~97% of steady-state.
The same math can be read in reverse for washout: after about 4–5 half-lives following a last dose, a similar 94–97% of the drug has been cleared in simple models.
4. Linking timing to number of doses
To give a more intuitive feel, the tool divides the “time to near steady-state” by your dosing interval in hours. That produces a rough count of how many usual doses you might receive before you are near steady-state. For example, if it takes about 96 hours to get close to steady-state and you take the medicine every 24 hours, that corresponds to about 4 doses.
5. Optional “relative amount” when a dose is entered
If you add a typical single dose in milligrams, the planner applies a very simplified accumulation factor to describe a relative amount in the body near steady-state. This is just a teaching tool to show that repeated doses stack up over time; it is not a blood level, not a therapeutic range and not a lab substitute.
In real pharmacokinetics, many more parameters matter: volume of distribution, clearance, absorption rate, protein binding, active metabolites and more. That is why any real-world dosing, tapering or switching plan has to be made by qualified clinicians, even if the half-life numbers in this planner look reassuring.
References and further reading on half-life and steady-state
These resources explain half-life, steady-state and the usual “4–5 half-life” timing idea in more depth:
- StatPearls — Elimination Half-Life of Drugs — outlines how half-life relates to drug elimination and notes that most of a drug is typically cleared after about 4–5 half-lives in simple models.
- Certara — Understanding Steady-State Pharmacokinetics — describes how steady-state depends on elimination half-life and shows that roughly 4–5 half-lives are usually needed to get close to steady-state with regular dosing.
- Medical News Today — Drug half-life: Meaning, why it matters, and more — gives a patient-friendly overview of drug half-life, factors that change it, and why 94–97% of a drug is often cleared after around 4–5 half-lives.
Use these as background reading only. For decisions about your own medicines, timing and lab monitoring, work directly with your prescriber, pharmacist and wider care team.