Medication Half-Life Calculator

Half-life determines how often you need to take medication to maintain therapeutic levels. Short half-life drugs (2-6 hours) require frequent dosing - often 3-4 times daily. Longer half-life medications (24+ hours) can be taken once daily or less. Understanding this helps you plan dosing schedules and understand why missing a dose matters more for some medications than others.

Steady state occurs when the amount of drug entering your body equals the amount being eliminated. After one half-life, 50% of the first dose remains when you take the second dose. This accumulation continues until equilibrium is reached at approximately 4-5 half-lives (94-97% of steady state). For a medication with a 24-hour half-life, it takes about 5 days to reach full therapeutic effect. This is why antidepressants (which often have long half-lives) take weeks to work.

Yes, significantly. Half-life can be affected by age (elderly often have slower elimination), kidney function (many drugs eliminated by kidneys), liver function (where most drugs are metabolized), genetics (some people lack enzymes to break down certain drugs), body composition, other medications (drug interactions can speed up or slow down elimination), and disease states. Published half-lives are population averages - your individual half-life may be shorter or longer.

Some medications are broken down into active metabolites that also have therapeutic effects and their own half-lives. For example, diazepam (Valium) has a 48-hour half-life, but its active metabolite has a half-life of up to 100 hours. The calculator shows elimination of the parent drug only. Always check drug information for metabolite activity - the effects may last much longer than the parent drug's half-life suggests.

Drug tests can detect substances long after therapeutic effects wear off. While 5 half-lives represents ~97% elimination, sensitive tests can detect trace amounts for longer - especially for long half-life medications or those with active metabolites. Additionally, some drugs accumulate in hair, fat tissue, or other compartments where they're released slowly. If you need to know detection windows for employment, legal, or sports testing, consult the specific testing protocol or a toxicology specialist.

Steady state means drug levels are stable because intake equals elimination. If you stop taking the medication, elimination continues but intake stops, so levels drop by 50% each half-life. For conditions requiring consistent drug levels (like blood pressure, seizures, or depression), this drop can cause symptoms to return or even rebound effects. Some medications require gradual tapering to avoid withdrawal. Always consult your healthcare provider before stopping any medication, even if you feel better.