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Drug Half Life Calculator

Drug Half Life Calculator

Estimate the exponential decay of a drug in the body by just providing the dosage and the half-life of the drug to the calculator.







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Use the drug half life calculator to determine the amount of a drug still present in a patient’s body when half time and dosage are known.

What Does Half Life Mean In Drugs?

The half life of a drug is the estimation of the time that the amount of a drug takes to be reduced by one-half. The symbol t1/2 is used for the half-life. 

Patient-Specific Variables That May Affect Half-Life:

  • Age
  • Blood circulation
  • Diet
  • Excessive fluid
  • Gender
  • Kidney function
  • Liver function
  • Obesity
  • Smoking

What is the formula for calculating half-life?:

The formula that is used to find the half life of drug is as follows:

\(\ Dosage(t) =\ Dosage(0)\times\ 0.5^{(\frac{t}{T})}\)


  • T is the representative of the Half-life of a drug;
  • t is the Time that has passed from the administration of the drug;
  • Dosage(t) – The drug amount present in a patient’s body
  • Dosage(0) shows the drug quantity administered

How do you Calculate the Half-life of a Drug?

Go through the steps outlined below:

  • Measure the quantity of the drug dosage
  • Determine the half-life time 
  • Put these values in the half-life formula as we have done in the following example. 


Suppose you have the following values:

  • Half Life Drug = 1 minute
  • Dosage = 10 g 

Find the half life?


Convert Grams to milligrams:

\(\ Dosage =\ 10\ \times\ 1000 =\ 10000\ mg\)

Put the given values in the half life drugs formula:

\(\ Dosage(t) =\ Dosage(0)\times\ 0.5^{(\frac{t}{T})}\)

\(\ Dosage(t) =\ 10000\ mg\times\ 0.5^{(\frac{1}{1})}\)

\(\ Dosage(t) = 5000\ mg\)

Half-Life of Drugs Chart:

In the following table, we have mentioned some most commonly used substances and their half life.

Generic Name Brand Name Examples Half-life (T1/2*)
Alprazolam Xanax 6-12 hours
Amiodarone Pacerone 15-142 days
Amphetamine Adderall, Dexedrine 10-12 hours
Atenolol Tenormin 6-7 hours
Clonazepam Klonopin 18-50 hours
Cocaine 50 minutes
Diazepam Valium 20-100 hours
Donepezil Aricept 70 hours
Dutasteride Avodart 5 weeks
Erenumab Aimovig 28 days
Fluoxetine Prozac 2-4 days
Lead 28-36 days
Mercury 65 days
Methamphetamine Desoxyn 6.4-15 hours
Methylphenidate Concerta, Ritalin 2-3 hours
Plutonium 40 years (liver), 100 years (bone)
Phenytoin Dilantin, Phenytek 7-42 hours
Tetrahydrocannabinol (cannabis/marijuana) Infrequent users: 1.3 days
Regular users: 13 days


Is Half Life The Same As Drug Duration?

No, The time that the drug takes to be eliminated from the body is the half life while the drug duration is the total time the drug remains inside the body. 

How many half-lives does it take to get rid of medication?

A drug approximates takes 4 to 5 half lives to be eliminated from the body. When 4 to half-lives are passed the amount of the drug goes below the clinically relevant concentration and is normally considered eliminated. If you double the amount the elimination time also increases. 

References: What does drug half-life mean?