Pharmacokinetic Half-Life & Dosing Interval Calculator
Explore basic pharmacokinetics with a simple half-life and dosing interval calculator. Estimate half-life from k or from two time–concentration points, see how much drug remains after a given interval, and compute times to 90%, 95%, or 99% elimination in a one-compartment, first-order model.
Understanding Pharmacokinetic Half-Life
Key Concepts
- Half-life (t½): The time required for the amount or concentration of a substance to fall to 50% of its initial value. For example, if a drug has a half-life of 4 hours, after 4 hours only half remains.
- Elimination rate constant (k): A parameter that describes how quickly a substance is eliminated. Related to half-life by:
t½ = ln(2) / k ≈ 0.693 / k - First-order kinetics: The rate of elimination is proportional to the amount present. This gives exponential decay:
C(t) = C₀ × e^(-kt)
Half-Life Multiples
Understanding how much remains after multiple half-lives:
| Half-lives | % Remaining | % Eliminated |
|---|---|---|
| 1 | 50% | 50% |
| 2 | 25% | 75% |
| 3 | 12.5% | 87.5% |
| 4 | 6.25% | 93.75% |
| 5 | 3.125% | 96.875% |
Rule of thumb: After ~5 half-lives, approximately 97% of a substance has been eliminated.
Dosing Interval Intuition
- If doses are repeated before much elimination occurs (short interval relative to half-life), drug levels can accumulate.
- If the interval is long compared to half-life, levels drop significantly between doses.
- The "theoretical interval" shown in this tool is simply the time needed to reach a chosen fraction remaining—it is NOT a clinical dosing recommendation.
Important Limitations
- • This model ignores absorption and distribution phases.
- • Real drugs often follow multi-compartment kinetics, not this simple one-compartment model.
- • Saturable clearance, protein binding, and organ function are not considered.
- • Patient variability (age, weight, genetics, disease state) is not accounted for.
- • This is NOT suitable for therapeutic drug monitoring or clinical dosing decisions.
Educational Use Only
This tool is designed for educational and research training purposes only. It provides simplified pharmacokinetic calculations to help build intuition about half-life and elimination. Do not use these outputs to start, stop, or adjust medications in real patients. Always consult qualified healthcare professionals and use validated PK/PD software for clinical decisions.
Frequently Asked Questions
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