Visualize how a drug accumulates in your system with repeated doses and clears after stopping. The chart shows concentration rising to steady-state during dosing, then exponential decay after the stop day.
Half-life is the time it takes for the concentration of a drug in your body to reduce by half. This is a key concept in pharmacokinetics—the study of how drugs move through your body over time.
When you take a medication regularly, each new dose adds to what remains from previous doses. If you take a drug with a 24-hour half-life once daily, by the time your next dose arrives, about 50% of the previous dose remains. This creates a "stacking" effect where concentration gradually rises until it reaches a steady state—the point where the amount entering your system equals the amount being eliminated.
It typically takes about 4-5 half-lives to reach steady state. For a drug with a 24-hour half-life taken daily, this means roughly 4-5 days. The chart above shows this accumulation pattern, with the sawtooth pattern representing the rise after each dose and decay between doses.
When you stop taking a medication, the concentration follows an exponential decay curve. After each half-life period, the remaining concentration halves:
Understanding half-life helps explain why some medications need to be taken multiple times daily (short half-life) while others only once (long half-life). It also explains why it can take several days to feel the full effects of a new medication, and why effects may linger after you stop taking it.
This drug half-life calculator helps you visualize medication concentration in your body over time. Here's how to get the most accurate results:
Use the dropdown menu to select from common medications with pre-filled values, or choose "Custom / Manual Entry" to input your own values. The dropdown includes medications across categories like hormones, peptides, pain relievers, antidepressants, and supplements.
Dosage (mg): Enter the amount you take per dose. This affects the peak concentration but not the shape of the curve.
Half-life (hours): This is how long it takes for half the drug to be eliminated. You can find this information in medication package inserts or ask your pharmacist.
Dose Interval: How often you take the medication. Common intervals are 24 hours (once daily), 12 hours (twice daily), or 8 hours (three times daily).
Day Stopped Taking: The day you stop or plan to stop the medication. The chart will show accumulation before this day and elimination after.
Total Days to Show: Extend this to see the full elimination curve. A drug is considered effectively cleared after 5 half-lives.
The green section shows the dosing phase where medication accumulates with each dose. The orange section shows elimination after you stop. The purple dashed line represents the average concentration, which is useful for understanding your typical drug level throughout the day.
Half-life is the time required for the concentration of a drug in your bloodstream to decrease by 50%. For example, if you take 100mg of a drug with a 4-hour half-life, after 4 hours you'll have approximately 50mg remaining, after 8 hours about 25mg, and so on.
Half-life depends on how a drug is metabolized and eliminated from your body. Factors include:
Steady state occurs when the amount of drug entering your body equals the amount being eliminated. At steady state, drug concentrations fluctuate within a consistent range between doses. It typically takes 4-5 half-lives to reach steady state. For a drug with a 24-hour half-life taken once daily, this means about 4-5 days.
The sawtooth pattern reflects real drug kinetics: concentration spikes after each dose, then gradually decreases as the drug is eliminated. The peaks represent maximum concentration (Cmax) shortly after dosing, while the troughs represent minimum concentration (Cmin) just before the next dose.
A drug is considered effectively eliminated after 5-7 half-lives, when less than 1-3% remains. For a drug with a 24-hour half-life, this means about 5-7 days. However, trace amounts may be detectable longer depending on the sensitivity of testing methods.
Body weight can affect drug distribution and therefore apparent half-life. Larger individuals may have more tissue for fat-soluble drugs to distribute into. However, the half-life values used in this calculator are population averages and provide reasonable estimates for most people.
This calculator uses standard pharmacokinetic models and average half-life values. Individual variation exists due to genetics, age, liver function, kidney function, other medications, and health conditions. Use this as an educational tool, not for medical decision-making.
Half-life measures how long a drug stays in your body, while duration of effect is how long the drug produces its therapeutic action. These can differ significantly—a drug might have effects that last longer or shorter than its half-life suggests, depending on how it works.
Understanding these key terms will help you better interpret the calculator results:
The process by which a drug moves from its site of administration into the bloodstream. Oral medications must pass through the digestive system, while injections enter the blood more directly.
The fraction of an administered dose that reaches systemic circulation unchanged. Intravenous drugs have 100% bioavailability, while oral drugs may have lower bioavailability due to incomplete absorption or first-pass metabolism in the liver.
The maximum concentration of a drug in the blood after a dose. This typically occurs within minutes to hours after taking a medication, depending on the route of administration and drug formulation.
The lowest concentration of a drug in the blood, typically measured just before the next scheduled dose. Monitoring trough levels is important for drugs with narrow therapeutic windows.
The volume of blood completely cleared of a drug per unit of time. Higher clearance means faster elimination. Clearance is affected by liver and kidney function.
The process by which a drug spreads from the bloodstream into body tissues. Drugs may concentrate in certain tissues based on their chemical properties.
The removal of a drug from the body through metabolism (usually in the liver) and excretion (usually through kidneys). First-order elimination, which this calculator models, means a constant fraction of the drug is eliminated per unit time.
A pattern where the rate of drug elimination is proportional to drug concentration. Most drugs follow first-order kinetics at therapeutic doses, resulting in the exponential decay curves shown in this calculator.
The range of drug concentrations that produces the desired effect without causing toxicity. Drugs with narrow therapeutic windows require careful dosing and monitoring.
A theoretical volume that relates the total amount of drug in the body to its blood concentration. Drugs with high Vd distribute extensively into tissues, while those with low Vd remain primarily in the blood.
Pharmacokinetics vary significantly between individuals. Factors that can affect how your body processes medications include:
This calculator uses a simplified one-compartment pharmacokinetic model with first-order elimination. It assumes:
Real drug behavior is often more complex, involving multiple compartments, variable absorption rates, and sometimes non-linear kinetics at high doses.
This calculator is for educational purposes only. You should consult a healthcare professional for: