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Drug Half-Life Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Greenblatt DJ. Elimination half-life of drugs: value and limitations; Annual Review of Medicine; 1985See 1 more source
Smith DA, Beaumont K, Maurer TS, Di L. Relevance of Half-Life in Drug Design.; Journal of Medicinal Chemistry; May 2018
Last updated: Feb 23, 2024

Our drug half-life calculator is an easy tool to discover the dosage of a drug that is still present in a patient's system. The half-life of a drug describes the process of its elimination.

Read on to discover the half-life of a drug and how to calculate the half-life of a medication. We'll also present plenty of useful examples.

This form of half-life is not to be mistaken with the decay of radioactive elements! To learn more about the principles of radioactive decay, visit the half-life calculator. ☢️

How to use the drug half-life calculator?

In order to use our 1/2 life calculator you'll need the following data:

  1. The half time of a given drug – this input can be either in minutes, hours, or days. The calculator will set the unit of the result automatically.

    If you don't know the half-life of the drug, take a look at our table below.

  2. Dosage – the amount of the drug administered at the very start. Learn more about the drug dosage calculation formula using the dosage calculator.

  3. Your results will be shown in a table that consists of at least 5 half-life cycles:

    • Time – the amount of time that has passed since the first administration of a drug;

    • Dosage – the amount of drug left in the patient's body at the given time; and

    • Concentration – the part of the original concentration still present in the patient's body.

What is the half-life of a drug?

In pharmacology, when we talk about the half-life of a drug, we usually mean the time it takes a drug's concentration in the blood plasma to reduce by half.

💡 The half-life of a drug describes the amount of time it will take half of the administered drug to be eliminated.

Why do we need it?

The half-life of a drug helps us assess the potency and toxicity of a drug over a given amount of time. If we know the elimination time, we can easily evaluate how often the drug should be administered to maximize its effect and minimize its adverse reactions.

The concentration of a drug at a given moment crucial in drugs with a narrow tolerance range, e.g., medicines that can prolong the patient's QT and consequently lead to heart arrhytmias.

🕒 The half-life of a drug is the complete opposite of its doubling time.

How to calculate a half-life of a drug?

Our drug elimination half-life calculator uses the following equation:

Dosage(t) = Dosage(0) × 0.5(t/T)


  • T – Half-life of a drug;
  • t – Time that has passed since the first, original administration of the drug;
  • Dosage(t) – Amount of drug present in a patient's body after time t; and
  • Dosage(0) – Amount of drug administered at the very beginning.

Half-life of medication – examples

Don't know the half-life of a given drug? Are you looking for an Adderall half-life calculator or any other specific drug calculations? We're here to help!




1-4 h


9-14 h


26-107 days


14-47 h


4 h


21-37 h


36-48 h


2-3 min


3-12 h


20-50 h


3-6 h


14-41 h


3-6 h


2-8 h


1-2 h


3-10 h


8 h


1-7 h


3-5 h


81-117 h


2-6 h


5-6 h


7-11 h

Valproic Acid

9-16 h


20-60 h

Want to explore appropriate doses for various medications such as Adderall and Lidocaine? Visit the Omni adderall dosage calculator and the lidocaine dose calculator.


What's the half-life of Adderall?

The half-life of Adderall is between 9 to 14 hours.

It means that after 14 hours, half of the amount you've taken will be already gone from your body.

What drug has the shortest half-life?

Adrenaline (epinephrine) has one of the shortest half-lives of only 2-3 minutes.

Adrenaline is a life-saving medication used in severe allergic reactions; due to its short half-life, its dose sometimes has to be repeated.

What drug has the longest half-life?

Amiodarone finds its place amongst the drugs with the longest half-life. It can take even 107 days(!) for our body to get rid of only half of a dose we've taken.

Amiodarone is a drug used in some heart arrhythmias and in resuscitation.

How much Adderall will be in my bloodstream after 6 hours?

Let's assume that the Adderall's half-life is 12 hours and that the patient took 1 g of the drug. It's passed 6 hours since the original administration of the drug. This is how you calculate its current level:

  1. Divide the time that's passed by the drug's half-life (6 / 12 = 0.5).
  2. Raise a half to the power of the result from step 1 (0.50.5 = 0.707).
  3. Multiply by the initial drug dose (1 g × 0.707).
  4. So, the amount of the drug after 6 hours is 0.71 g.

After 6 hours, the patient still has ~0.71 g of Adderall in their bloodstream.

Łucja Zaborowska, MD, PhD candidate
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