Drug HalfLife Calculator
Our drug halflife calculator is an easy tool to discover the dosage of a drug that is still present in a patient's system. The halflife of a drug describes the process of its elimination.
Read on to discover the halflife of a drug and how to calculate the halflife of a medication. We'll also present plenty of useful examples.
This form of halflife is not to be mistaken with the decay of radioactive elements! To learn more about the principles of radioactive decay, visit the halflife calculator. ☢️
How to use the drug halflife calculator?
In order to use our 1/2 life calculator you'll need the following data:

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 halflife of the drug, take a look at our table below.

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

Your results will be shown in a table that consists of at least 5 halflife 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 halflife of a drug?
In pharmacology, when we talk about the halflife of a drug, we usually mean the time it takes a drug's concentration in the blood plasma to reduce by half.
💡 The halflife 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 halflife 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 halflife of a drug is the complete opposite of its doubling time.
How to calculate a halflife of a drug?
Our drug elimination halflife calculator uses the following equation:
Dosage(t) = Dosage(0) × 0.5^{(t/T)}
where:
 T – Halflife 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.
Halflife of medication – examples
Don't know the halflife of a given drug? Are you looking for an Adderall halflife calculator or any other specific drug calculations? We're here to help!
Drug  Halflife 

Acetaminophen  14 h 
Adderall  914 h 
Amiodarone  26107 days 
Carbamazepine  1447 h 
Ciprofloxacin  4 h 
Diazepam  2137 h 
Digoxin  3648 h 
Epinephrine  23 min 
Fentanyl  312 h 
Fluconazole  2050 h 
Ganciclovir  36 h 
Haloperidol  1441 h 
Heparin  36 h 
Ketoconazole  28 h 
Lidocaine  12 h 
Methotrexate  310 h 
Metronidazole  8 h 
Morphine  17 h 
Oxycodone  35 h 
Phenobarbital  81117 h 
Propranolol  26 h 
Streptomycin  56 h 
Tetracycline  711 h 
Valproic Acid  916 h 
Warfarin  2060 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.
FAQ
What's the halflife of Adderall?
The halflife 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 halflife?
Adrenaline (epinephrine) has one of the shortest halflives of only 23 minutes.
Adrenaline is a lifesaving medication used in severe allergic reactions; due to its short halflife, its dose sometimes has to be repeated.
What drug has the longest halflife?
Amiodarone finds its place amongst the drugs with the longest halflife. 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 halflife 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:
 Divide the time that's passed by the drug's halflife (6 / 12 = 0.5).
 Raise a half to the power of the result from step 1 (0.5^{0.5} = 0.707).
 Multiply by the initial drug dose (1 g × 0.707).
 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.