# Medicine Half Life Calculator

Determines the concentration decrease of any substance, when half time and dosage are known.

In the text below the tool you can find an explanation on how the calculator works and a table with the half life of the most common active substances.

The medicine half life calculator can be used to follow the plasmatic concentration decrease (in percentage) for any substance, when dosage and half time are input.

Half life estimation is especially useful when trying to keep constant levels of medicine in the body or when trying to avoid pharmacological interactions between substances.

For a dosage of 35 mg with a half life of 1.5 hours, the concentration decreases as follows:

 After… Dosage (mg) Concentration 1.5 hrs 17.5 mg 50% 3 hrs 8.8 mg 25% 4.5 hrs 4.4 mg 12.5% 6 hrs 2.2 mg 6.25% 7.5 hrs 1.1 mg 3.125% 9 hrs 0.5 mg 1.562% 10.5 hrs 0.3 mg 0%

Dosage
Half time
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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.

## Drug half life explained

This medicine half life calculator requires the drug dosage (measured in mg, µg or g) and the half time (estimated in hours or minutes).

The decrease in plasma concentration is followed at 7 stages:

■ 50%;

■ 25%;

■ 5%;

■ 25%;

■ 125%;

■ 562%;

■ 0%.

The half time or life of a dose is defined as the period of time after administration, in either hours of minutes, in which the dosage reaches half of its concentration in the plasma.

Pharmacologically this is expressed as the Peak minus Through concentrations, divided by the interval.

Biologically, this means the elimination half time of the medicine and it is affected by factors such as receptor interactions or metabolic activity.

In order to produce the desired effect, the active substance needs to remain at a constant level for the duration of the treatment. Half life is used by professionals to establish dosage and administration schedule.

## Substances and their half time

The following table introduces the half time of most common active substances:

 Substance Time (hrs unless otherwise stated) Substance Time (hrs) Acetaminophen 1 - 4 Indinavir 1 - 3 Acyclovir 2 - 3 Isoniazid 1 - 2 Amikacin 2 - 3 Ketoconazole 2 - 8 Amiodarone 26 - 107 days Lidocaine 1 - 2 Amphetamine 7 - 34 Lithium 14 - 30 Bismuth 21 - 72 days Methotrexate 3 - 10 Caffeine 3 - 12 Metronidazole 8 Carbamazepine 14 - 47 Morphine 1 - 7 Cefuroxime 1 - 2 Oxcarbazepine 1 - 2 Cephalexin 50 - 60 min Oxycodone 3 - 5 Chlordiazepoxide 6 - 25 Phenobarbital 81 - 117 Chlorpromazine 7 - 119 Phenytoin 18 - 22 Ciprofloxacin 4 Primidone 3 - 7 Clarithromycin 3 - 4 Procainamide 2 - 5 Clonazepam 19 - 60 Propafenone 2 - 10 Cyclosporine 6 - 27 Propranolol 2 - 6 Diazepam 21 - 37 Pyrazinamide 10 Digitoxin 4 - 10 days Quinidine 5 - 9 Digoxin 36 - 48 Rifabutin 16 - 69 Doxepin 8 - 25 Rifampin 2 - 3 Doxycycline 18 - 22 Ritonavir 3 - 5 Fentanyl 3 - 12 Salicylate 2 - 3 Fluconazole 20 - 50 Streptomycin 5 - 6 Gabapentin 5 - 7 Tetracycline 7 - 11 Ganciclovir 3 - 6 Theophylline 7 - 11 Gentamicin 1 - 3 Trimethoprim 8 - 10 Haloperidol 14 - 41 Valproic Acid 9 - 16 Heparin Anti-Xa 3 - 6 Vancomycin 3 - 8 Imipramine 6 - 20 Warfarin 20 - 60

## References

1) Hammett-Stabler CA, Dasgupta A, eds. (2007) Therapeutic Drug Monitoring Data: A Concise Guide. 3rd ed. Washington, DC: AACC Press.

2) Garg U, Jacobs DS, Grady HJ, et al. Therapeutic drug monitoring. In: Jacobs DS, Oxley DK, Demott WR, eds. (2001) Jacobs & Demott Laboratory Test Handbook. 5th ed. Cleveland, Ohio: Lexi-Comp, Inc; 731-771.

Specialty: Pharmacology

Objective: Determination

Type: Calculator

No. Of Variables: 2

Article By: Denise Nedea

Published On: May 22, 2017 · 07:17 AM

Last Checked: May 22, 2017

Next Review: May 22, 2023