Drug Half-Life Calculator

Drug Half-Life Calculator. A drug's half-life tells you how long it takes for half the medication to leave your body — useful for understanding dosing intervals, timing, and clearance. Enter your initial dose, the drug's half-life (in minutes, hours, or days), and the time elapsed since taking it to see the remaining dose in mg. Secondary outputs include percentage remaining, percentage eliminated, and the number of half-lives elapsed. Also try the calculate Drug Dosage.

Initial Dose *

The starting amount of medication taken

Half-Life *

Time it takes for drug concentration to reduce by 50%

Half-Life Unit *

Time Elapsed *

How much time has passed since taking the medication

Time Unit *

Results

Remaining Dose

Percentage Remaining

Percentage Eliminated

Half-Lives Elapsed

Results Table

What is the half-life of a drug?

The half-life of a drug is the time it takes for the concentration of that drug in the body to be reduced by exactly 50%. It's a measure of how quickly the body eliminates or metabolizes a medication. See also our Dosage Calculator (mg/kg).

How do you calculate the half-life of a drug?

Drug half-life is calculated using the formula: Remaining amount = Initial dose × (0.5)^(time elapsed / half-life). This exponential decay formula shows how drug concentration decreases over time.

How many half-lives does it take to eliminate a drug?

Most drugs are considered to have negligible effects after 4-5 half-lives, when about 94-97% of the drug has been eliminated. However, traces may still be detectable in drug tests.

What drug has the shortest half-life?

Some drugs have very short half-lives, such as adenosine (less than 10 seconds) and some anesthetics like propofol (2-24 minutes). These require continuous infusion or frequent dosing to maintain therapeutic levels.

What drug has the longest half-life?

Some drugs have extremely long half-lives, such as amiodarone (25-100 days) and fluoxetine (4-6 days, but active metabolites can last weeks). These drugs remain in the system for extended periods.

Why is drug half-life important in medicine?

Half-life determines dosing frequency, helps predict drug interactions, guides withdrawal protocols, and helps maintain therapeutic drug levels. It's crucial for both efficacy and safety.

What factors affect drug half-life?

Patient factors include age, kidney and liver function, body composition, and genetics. Drug factors include molecular structure, protein binding, and route of elimination. Disease states can also significantly alter half-life.

Can drug half-life vary between individuals?

Yes, half-life can vary significantly between individuals due to differences in metabolism, organ function, age, weight, and genetic factors. This is why therapeutic drug monitoring is sometimes necessary. You might also find our Creatinine Clearance — Clearance Rate useful.