# Half Life Calculator

Determines the elimination constant and half time of medication based on peak and trough concentrations.

In the text below the form you can find more information about medicine half life and the formula used to calculate it.

The half life calculator uses the peak and trough concentrations and the time interval to determine the half life of any medicine and the elimination constant. The concentrations can be input in mcg/mL or mg/mL.

The two formulas used are:

Half life = 0.693 / (Peak c - Trough c) / Time interval

Elimination constant = (Peak c - Trough c) / Time interval

Peak concentration:*
Trough concentration:*
Time interval:*
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Steps on how to print your input & results:

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.

## Variables and formula

The above calculator determines the half life time in hours and the elimination constant (in mcg/mL/hrs) of any medicine, when the following three variables are known:

■ Peak concentration: or Cmax is the maximum serum concentration that a drug achieves during the time between a first and second dose administration.

■ Trough concentration: is the lowest concentration of the drug before the next dose is administered.

■ Time interval: time between peak and trough concentration.

The following two formulas are used to determine the intermediary elimination constant and ultimately the half time.

Elimination constant = (Peak c - Trough c) / Time interval

Half life = 0.693 / (Peak c - Trough c) / Time interval

## Half life explained

This indicates the amount of time a substance requires to release half of its initial concentration during metabolism or elimination through normal processes.

This is a measure of the rapidity of pharmacokinetic processes of the medicine in the plasma and is symbolized as t½.

Half life is dependent on the peak and trough concentrations, the former being obtained when the medicine is entirely distributed in the system.

The following table introduces some of the most common active substances and their half lives in hour or minutes:

 Clonazepam 18 - 50 hours Diazepam 20 - 100 hours Flurazepam 0.8 - 4.2 days Ibuprofen 1.3 - 3 hours Methadone 15 to 72 hours Morphine 2 - 3 hours Norepinephrine 2 minutes Oxaliplatin 14 minutes Paracetamol 1 - 4 hours Phenytoin 12 - 42 hours Salbutamol 1.6 hours Zaleplon 1 - 2 hours

## References

1. Moyer TP, Shaw LM. Therapeutic drugs and their management. In: Burtis C, Ashwood E, Bruns D. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 4th ed. Philadelphia, PA: Saunders; 2005.

2. Pippenger CE. Principles of therapeutic drug monitoring. In: Wong SHY. ed. Therapeutic Drug Monitoring and Toxicology by Liquid Chromatography. Boca Raton, FL: CRC Press; 1985.

Specialty: Pharmacology

Objective: Determination

Type: Calculator

No. Of Variables: 3

Article By: Denise Nedea

Published On: June 19, 2017

Last Checked: June 19, 2017

Next Review: June 19, 2023