Enter your Initial Dose, the drug's Half-Life (in your chosen Half-Life Unit), and the Time Elapsed since taking it — the Drug Half-Life Calculator will show you the Remaining Dose in your system, plus the Percentage Remaining, Percentage Eliminated, and exactly how many Half-Lives have passed.
Remaining Dose
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Percentage Remaining
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Percentage Eliminated
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Half-Lives Elapsed
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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.
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.
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.
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.
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.
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.
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.
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.