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.
Last Updated: November 17, 2025. This content is regularly reviewed to ensure accuracy and alignment with current pharmacokinetics principles.
Understanding Pharmacokinetic Half-Life and Dosing Intervals: Essential Calculations for Drug Elimination and Dosing
Pharmacokinetic half-life (t½) is the time required for the amount or concentration of a drug to decrease to 50% of its initial value under first-order elimination. Understanding half-life is crucial for students studying pharmacology, pharmacokinetics, drug dosing, and biomedical research, as it explains how drugs are eliminated from the body, how dosing intervals affect drug levels, and how to estimate elimination times. Half-life calculations appear in virtually every pharmacokinetics protocol and are foundational to understanding drug elimination.
Key components of pharmacokinetic half-life include: (1) Half-life (t½)—time for 50% elimination, (2) Elimination rate constant (k)—describes elimination rate, related to half-life by t½ = ln(2)/k ≈ 0.693/k, (3) First-order kinetics—elimination rate proportional to amount present, giving exponential decay C(t) = C₀ × e^(-kt), (4) Dosing intervals—time between doses, affecting drug accumulation, (5) Fraction remaining—proportion of drug remaining after a given time. Understanding these components helps you see why each is needed and how they work together.
One-compartment model assumes the body acts as a single, well-mixed compartment with first-order elimination. This simplified model provides useful intuition about half-life concepts, though many real drugs follow more complex multi-compartment kinetics. Understanding this model helps you see how basic pharmacokinetic principles work and why it's commonly taught in introductory pharmacology courses.
Dosing intervals determine how frequently drugs are administered. If intervals are short relative to half-life, drug levels can accumulate. If intervals are long, levels drop significantly between doses. The theoretical interval for a target fraction remaining is calculated as τ = -ln(f)/k, where f is the fraction remaining. Understanding dosing intervals helps you see how they affect drug levels and why proper interval selection is important.
Elimination milestones are commonly cited in pharmacokinetics: 90% elimination (10% remaining) takes about 3.3 half-lives, 95% elimination (5% remaining) takes about 4.3 half-lives, 99% elimination (1% remaining) takes about 6.6 half-lives. After approximately 5 half-lives, about 97% of a drug has been eliminated. Understanding these milestones helps you estimate drug washout periods and elimination times.
This calculator is designed for educational exploration and practice. It helps students master pharmacokinetic half-life by calculating half-life from k or vice versa, estimating k from time-concentration points, determining fraction remaining after intervals, and calculating elimination times. The tool provides step-by-step calculations showing how exponential decay and first-order elimination work. For students preparing for pharmacology exams, pharmacokinetics courses, or drug dosing labs, mastering half-life is essential—these concepts appear in virtually every pharmacokinetics protocol and are fundamental to understanding drug elimination. The calculator supports comprehensive analysis (half-life calculation, interval analysis, elimination times), helping students understand all aspects of pharmacokinetic half-life.
Critical disclaimer: This calculator is for educational, homework, and conceptual learning purposes only. It helps you understand pharmacokinetic theory, practice half-life calculations, and explore how drug elimination works. It does NOT provide instructions for actual drug dosing decisions, which require proper training, validated PK/PD software, patient-specific factors, and adherence to clinical guidelines. Never use this tool to determine actual dosing schedules, adjust medications, or make clinical decisions without proper healthcare professional consultation and validated software. Real-world drug dosing involves considerations beyond this calculator's scope: multi-compartment kinetics, absorption and distribution phases, saturable clearance, protein binding, patient variability (age, weight, renal/hepatic function), drug interactions, therapeutic windows, and clinical judgment. Use this tool to learn the theory—consult qualified healthcare professionals and validated software for practical applications.
Understanding the Basics of Pharmacokinetic Half-Life
What Is Pharmacokinetic Half-Life and Why Does It Matter?
Half-life (t½) is the time required for the amount or concentration of a drug to decrease to 50% of its initial value under first-order elimination. Understanding half-life helps you see why it's fundamental to pharmacokinetics, drug dosing, and understanding drug elimination.
How Do You Calculate Half-Life from Elimination Rate Constant?
Half-life is calculated from the elimination rate constant as: t½ = ln(2) / k ≈ 0.693 / k. For example, if k = 0.0866 per hour, then t½ = 0.693 / 0.0866 ≈ 8 hours. Understanding this relationship helps you convert between half-life and elimination rate constant.
How Do You Calculate Elimination Rate Constant from Half-Life?
Elimination rate constant is calculated from half-life as: k = ln(2) / t½ ≈ 0.693 / t½. For example, if t½ = 8 hours, then k = 0.693 / 8 ≈ 0.0866 per hour. Understanding this helps you convert between half-life and elimination rate constant.
How Do You Estimate k from Two Time-Concentration Points?
From two time-concentration points (t₁, C₁) and (t₂, C₂), assuming log-linear elimination: k = -slope = -(ln(C₂) - ln(C₁)) / (t₂ - t₁). This assumes both points are in the elimination phase (after absorption and distribution). Understanding this helps you estimate k from experimental data.
How Do You Calculate Fraction Remaining After a Dosing Interval?
Fraction remaining after interval τ is calculated as: Fraction Remaining = e^(-k × τ). For example, if k = 0.0866 per hour and τ = 8 hours, fraction remaining = e^(-0.0866 × 8) ≈ 0.5 (50%). Understanding this helps you see how much drug remains after a dosing interval.
How Do You Calculate Theoretical Dosing Interval for Target Fraction Remaining?
Theoretical interval for target fraction remaining f is calculated as: τ = -ln(f) / k. For example, if f = 0.25 (25% remaining) and k = 0.0866 per hour, τ = -ln(0.25) / 0.0866 ≈ 16 hours. Understanding this helps you see how interval relates to fraction remaining.
How Do You Calculate Time to Reach Specific Elimination Percentages?
Time to reach fraction eliminated (1 - f_remaining) is calculated as: Time = -ln(f_remaining) / k. For example, for 90% elimination (10% remaining), time = -ln(0.1) / k ≈ 3.32 × t½. Understanding this helps you estimate elimination times for specific percentages.
How to Use the PK Half-Life & Dosing Interval Calculator
This interactive tool helps you analyze pharmacokinetic half-life by calculating half-life from k or vice versa, estimating k from time-concentration points, and determining fraction remaining and elimination times. Here's a comprehensive guide to using each feature:
Step 1: Choose Primary Parameter
Select how you want to input pharmacokinetic data:
Half-Life
Enter half-life directly (e.g., 8 hours). The calculator computes k = ln(2) / t½.
Elimination Rate Constant
Enter k directly (e.g., 0.0866 per hour). The calculator computes t½ = ln(2) / k.
Time-Concentration Pair
Enter two time-concentration points (e.g., (2h, 10 mg/L) and (10h, 2 mg/L)). The calculator estimates k from log-linear fit, assuming both points are in elimination phase.
Step 2: Enter Dosing Interval (Optional)
If you want to analyze a specific dosing interval:
Dosing Interval Hours
Enter the time between doses (e.g., 8 hours). The calculator computes fraction remaining and fraction eliminated after this interval.
Step 3: Set Target Fraction Remaining (Optional)
If you want to calculate theoretical interval for a target fraction:
Target Fraction Remaining
Enter desired fraction remaining at trough (0 to 1, e.g., 0.25 for 25%). The calculator computes theoretical interval τ = -ln(f)/k.
Example: Half-life = 8 hours, interval = 8 hours, target = 0.25
Input: t½ = 8h, τ = 8h, f = 0.25
Output: k ≈ 0.0866/h, fraction remaining ≈ 0.5 (50%), theoretical interval for 25% ≈ 16h
Explanation: Calculator computes k from t½, calculates fraction remaining after interval, determines theoretical interval for target fraction.
Step 4: Calculate and Review Results
Click "Calculate" to get your results:
View Calculation Results
The calculator shows: (a) Half-life and elimination rate constant, (b) Fraction remaining after 1, 2, 3 half-lives, (c) Fraction remaining/eliminated after dosing interval, (d) Theoretical interval for target fraction, (e) Time to reach 90%, 95%, 99% elimination, (f) Summary and notes.
Tips for Effective Use
- Use half-life input for known half-life values (most common).
- Use elimination rate constant input if you have k values from literature or calculations.
- Use time-concentration pair input only if both points are in the elimination phase (after absorption/distribution).
- Remember that theoretical intervals are for educational intuition only, not clinical dosing recommendations.
- All calculations assume one-compartment, first-order elimination model.
- All calculations are for educational understanding, not actual drug dosing decisions.
Formulas and Mathematical Logic Behind Pharmacokinetic Half-Life
Understanding the mathematics empowers you to calculate half-life values on exams, verify calculator results, and build intuition about drug elimination.
1. Fundamental Relationship: Half-Life and Elimination Rate Constant
t½ = ln(2) / k ≈ 0.693 / k
Where:
t½ = half-life
k = elimination rate constant
ln(2) ≈ 0.693
Key insight: Half-life and elimination rate constant are inversely related. Larger k means faster elimination and shorter half-life. Understanding this helps you see how elimination rate affects half-life.
2. Exponential Decay Model
C(t) = C₀ × e^(-kt)
Where C(t) = concentration at time t, C₀ = initial concentration, k = elimination rate constant
Example: C₀ = 100, k = 0.0866/h, t = 8h → C(8) = 100 × e^(-0.0866×8) ≈ 50 (50% remaining)
3. Estimating k from Two Time-Concentration Points
k = -(ln(C₂) - ln(C₁)) / (t₂ - t₁)
Where (t₁, C₁) and (t₂, C₂) are two time-concentration points in elimination phase
Example: (2h, 10) and (10h, 2) → k = -(ln(2)-ln(10))/(10-2) = -(-1.609)/8 ≈ 0.201/h
4. Fraction Remaining After Dosing Interval
Fraction Remaining = e^(-k × τ)
Where τ = dosing interval, k = elimination rate constant
Example: k = 0.0866/h, τ = 8h → Fraction = e^(-0.0866×8) ≈ 0.5 (50%)
5. Theoretical Interval for Target Fraction Remaining
τ = -ln(f) / k
Where f = target fraction remaining (0 < f ≤ 1), k = elimination rate constant
Example: f = 0.25, k = 0.0866/h → τ = -ln(0.25)/0.0866 ≈ 16h
6. Time to Reach Specific Elimination Percentages
Time = -ln(f_remaining) / k
Where f_remaining = fraction remaining (e.g., 0.1 for 90% elimination), k = elimination rate constant
Example: 90% elimination (10% remaining), k = 0.0866/h → Time = -ln(0.1)/0.0866 ≈ 26.6h ≈ 3.32 × t½
7. Worked Example: Calculate Half-Life and Fraction Remaining
Given: k = 0.0866 per hour, dosing interval = 8 hours
Find: Half-life and fraction remaining after interval
Step 1: Calculate half-life
t½ = ln(2) / k = 0.693 / 0.0866 ≈ 8 hours
Step 2: Calculate fraction remaining after interval
Fraction = e^(-k × τ) = e^(-0.0866 × 8) ≈ e^(-0.693) ≈ 0.5 (50%)
Step 3: Calculate time to 90% elimination
Time = -ln(0.1) / 0.0866 ≈ 2.303 / 0.0866 ≈ 26.6 hours ≈ 3.32 × t½
Practical Applications and Use Cases
Understanding pharmacokinetic half-life is essential for students across pharmacology and pharmacokinetics coursework. Here are detailed student-focused scenarios (all conceptual, not actual drug dosing decisions):
1. Homework Problem: Calculate Half-Life from k
Scenario: Your pharmacology homework asks: "If k = 0.0866 per hour, what is the half-life?" Use the calculator: enter k = 0.0866. The calculator shows: t½ = 0.693 / 0.0866 ≈ 8 hours. You learn: how to use t½ = ln(2)/k to calculate half-life. The calculator helps you check your work and understand each step.
2. Lab Report: Understanding Fraction Remaining After Intervals
Scenario: Your pharmacokinetics lab report asks: "How much drug remains after 8 hours if t½ = 8 hours?" Use the calculator: enter t½ = 8h, interval = 8h. The calculator shows: Fraction remaining ≈ 0.5 (50%). Understanding this helps explain why after one half-life, 50% remains. The calculator makes this relationship concrete—you see exactly how interval affects fraction remaining.
3. Exam Question: Calculate Time to 90% Elimination
Scenario: An exam asks: "How long does it take to reach 90% elimination if t½ = 8 hours?" Use the calculator: enter t½ = 8h. The calculator shows: Time to 90% elimination ≈ 26.6 hours ≈ 3.32 × t½. This demonstrates how to calculate elimination times for specific percentages.
4. Problem Set: Estimate k from Time-Concentration Points
Scenario: Problem: "Estimate k from points (2h, 10 mg/L) and (10h, 2 mg/L)." Use the calculator: enter time-concentration pair mode, add both points. The calculator shows: k ≈ 0.201 per hour. This demonstrates how to estimate k from experimental data.
5. Research Context: Understanding Why Half-Life Matters
Scenario: Your pharmacology homework asks: "Why is half-life important in drug dosing?" Use the calculator: explore different half-lives and their effects on fraction remaining and elimination times. Understanding this helps explain why half-life determines dosing frequency, affects drug accumulation, guides washout periods, and supports dosing interval selection. The calculator makes this relationship concrete—you see exactly how half-life affects drug elimination and dosing.
Common Mistakes in Pharmacokinetic Half-Life Calculations
Half-life problems involve exponential decay, logarithmic calculations, and elimination rate constants that are error-prone. Here are the most frequent mistakes and how to avoid them:
1. Using Wrong Formula for Half-Life from k
Mistake: Using t½ = k / ln(2) instead of t½ = ln(2) / k, or forgetting to divide by k, leading to wrong half-life values.
Why it's wrong: Half-life and k are inversely related. Using k / ln(2) gives values that are k² times too large. For example, k = 0.0866/h, using 0.0866 / 0.693 = 0.125h (wrong, should be 8h).
Solution: Always remember: t½ = ln(2) / k ≈ 0.693 / k. The calculator does this automatically—observe it to reinforce the relationship.
2. Forgetting Negative Sign in k Estimation from Time-Concentration Points
Mistake: Using k = (ln(C₂) - ln(C₁)) / (t₂ - t₁) instead of k = -(ln(C₂) - ln(C₁)) / (t₂ - t₁), leading to negative k values.
Why it's wrong: Concentration decreases over time, so ln(C₂) < ln(C₁), giving negative slope. k must be positive, so we need the negative sign. For example, (2h, 10) and (10h, 2), using (ln(2)-ln(10))/(10-2) = -0.201/h (wrong sign, should be +0.201/h).
Solution: Always use: k = -(ln(C₂) - ln(C₁)) / (t₂ - t₁). The calculator does this correctly—observe it to reinforce k estimation.
3. Using Wrong Exponential Formula for Fraction Remaining
Mistake: Using Fraction = e^(k × τ) instead of e^(-k × τ), or forgetting the negative sign, leading to fractions > 1.
Why it's wrong: Drug concentration decreases over time, so fraction remaining must decrease. Using e^(k × τ) gives values > 1 (impossible). For example, k = 0.0866/h, τ = 8h, using e^(0.0866×8) = 2.0 (wrong, should be 0.5).
Solution: Always use: Fraction = e^(-k × τ). The calculator does this correctly—observe it to reinforce exponential decay.
4. Wrong Formula for Theoretical Interval
Mistake: Using τ = ln(f) / k instead of τ = -ln(f) / k, leading to negative intervals.
Why it's wrong: Since f < 1, ln(f) is negative. We need -ln(f) to get positive τ. Using ln(f)/k gives negative values. For example, f = 0.25, k = 0.0866/h, using ln(0.25)/0.0866 = -16h (wrong, should be +16h).
Solution: Always use: τ = -ln(f) / k. The calculator does this correctly—observe it to reinforce interval calculation.
5. Not Recognizing That Time-Concentration Points Must Be in Elimination Phase
Mistake: Using time-concentration points from absorption or distribution phases, leading to invalid k estimates.
Why it's wrong: k estimation assumes log-linear elimination. Points from absorption/distribution phases don't follow this pattern, giving wrong k. For example, using points during absorption phase gives wrong k estimate.
Solution: Always ensure both points are in the elimination phase (after absorption/distribution). The calculator assumes this—use it to reinforce phase recognition.
6. Confusing Fraction Remaining with Fraction Eliminated
Mistake: Using fraction remaining when fraction eliminated is needed, or vice versa, leading to wrong interpretations.
Why it's wrong: Fraction remaining + fraction eliminated = 1. Using wrong fraction gives wrong interpretation. For example, 50% remaining means 50% eliminated, not 50% remaining means 50% remaining.
Solution: Always remember: Fraction Eliminated = 1 - Fraction Remaining. The calculator shows both—use it to reinforce the distinction.
7. Not Realizing That This Tool Doesn't Provide Clinical Dosing Recommendations
Mistake: Assuming the calculator provides actual dosing schedules, therapeutic recommendations, or clinical guidance.
Why it's wrong: This tool uses a simplified one-compartment model and doesn't account for absorption, distribution, multi-compartment kinetics, patient variability, drug interactions, or therapeutic windows. Real dosing requires validated PK/PD software and clinical judgment.
Solution: Always remember: this tool calculates half-life for educational purposes only. You must use validated software and healthcare professionals for actual dosing. The calculator emphasizes this limitation—use it to reinforce that simplified calculation and clinical dosing are separate steps.
Advanced Tips for Mastering Pharmacokinetic Half-Life
Once you've mastered basics, these advanced strategies deepen understanding and prepare you for complex half-life problems:
1. Understand Why Exponential Decay Models Elimination (Conceptual Insight)
Conceptual insight: First-order elimination means elimination rate is proportional to amount present. This gives exponential decay C(t) = C₀ × e^(-kt). Understanding this provides deep insight beyond memorization: exponential decay naturally arises from first-order kinetics.
2. Recognize Patterns: Longer Half-Life = Slower Elimination
Quantitative insight: Half-life and elimination rate are inversely related. Longer half-life means slower elimination (smaller k), shorter half-life means faster elimination (larger k). Understanding this pattern helps you predict elimination: t½ = 8h means slower elimination than t½ = 4h.
3. Master the Systematic Approach: k → t½ → Fraction → Interval
Practical framework: Always follow this order: (1) Determine k (from t½, direct input, or time-concentration points), (2) Calculate t½ = ln(2)/k, (3) Calculate fraction remaining = e^(-k×τ), (4) Calculate theoretical interval = -ln(f)/k. This systematic approach prevents mistakes and ensures you don't skip steps. Understanding this framework builds intuition about pharmacokinetic calculations.
4. Connect Half-Life to Drug Dosing Applications
Unifying concept: Half-life is fundamental to drug dosing (determining dosing frequency), therapeutic drug monitoring (predicting drug levels), drug interactions (estimating washout periods), and clinical pharmacology (understanding drug elimination). Understanding half-life helps you see why it determines dosing intervals, affects drug accumulation, guides washout periods, and supports dosing decisions. This connection provides context beyond calculations: half-life is essential for modern pharmacology.
5. Use Mental Approximations for Quick Estimates
Exam technique: For quick estimates: After 1 t½, 50% remains; after 2 t½, 25% remains; after 3 t½, 12.5% remains. 90% elimination ≈ 3.3 t½, 95% ≈ 4.3 t½, 99% ≈ 6.6 t½. Understanding approximate relationships helps you quickly estimate on multiple-choice exams and check calculator results.
6. Understand Limitations: This Tool Assumes One-Compartment Model
Advanced consideration: This calculator assumes: (a) One-compartment, well-mixed model, (b) First-order elimination, (c) No absorption/distribution phases, (d) No multi-compartment kinetics, (e) No saturable clearance or patient variability. Real systems may show these effects. Understanding these limitations shows why multi-compartment models and patient-specific factors are often needed, and why advanced methods are required for accurate work in research, especially for complex drugs or non-standard conditions.
7. Appreciate the Relationship Between Half-Life and Dosing Frequency
Advanced consideration: Half-life affects dosing frequency: (a) Short t½ = more frequent dosing needed, (b) Long t½ = less frequent dosing possible, (c) Interval ≈ t½ = moderate accumulation, (d) Interval >> t½ = significant drop between doses. Understanding this helps you design dosing regimens that use half-life effectively and achieve optimal therapeutic outcomes.
Limitations & Assumptions
• One-Compartment Model: This calculator uses a simplified one-compartment pharmacokinetic model with first-order elimination. It does not account for multi-compartment distribution, absorption phases, or non-linear kinetics.
• First-Order Elimination Only: Calculations assume constant elimination rate constant (k) independent of drug concentration. Drugs with saturable elimination (zero-order kinetics at high doses) require more complex models.
• No Patient Variability: The calculator does not account for individual patient factors including age, weight, organ function, genetic polymorphisms, or disease states that significantly affect pharmacokinetics.
• Elimination Phase Data Required: When estimating k from concentration-time points, both data points must be from the elimination phase. Using data from absorption or distribution phases produces invalid estimates.
Important Note: This calculator is strictly for educational and informational purposes only. It is NOT intended for clinical dosing decisions. Actual drug dosing requires professional medical guidance, validated pharmacokinetic software, patient-specific considerations, and appropriate therapeutic drug monitoring.
Sources & References
The pharmacokinetics principles and dosing interval concepts referenced in this content are based on authoritative pharmaceutical sciences sources:
- NCBI StatPearls - Pharmacokinetics - Comprehensive pharmacokinetics reference
- OpenStax Anatomy and Physiology - Free peer-reviewed textbook (Drug distribution and elimination)
- FDA Drug Development Resources - Pharmacokinetic parameters and drug interactions
- PubMed/NCBI - Clinical pharmacology and pharmacokinetics literature
- DrugBank Database - Drug half-life and pharmacokinetic data
This tool is for educational purposes only. Clinical dosing decisions require professional medical guidance and patient-specific considerations.
Frequently Asked Questions
What does pharmacokinetic half-life mean in this tool?
Half-life (t½) is the time required for the amount or concentration of a substance to decrease to 50% of its initial value. In this simple one-compartment model, it assumes exponential decay where the elimination rate is proportional to the amount present (first-order kinetics). After one half-life, 50% remains; after two, 25%; after three, 12.5%, and so on. Understanding this helps you see how half-life quantifies drug elimination and why it's fundamental to pharmacokinetics.
How are half-life and elimination rate constant related?
The elimination rate constant (k) and half-life (t½) are mathematically related by: t½ = ln(2) / k, which is approximately 0.693 / k. If you know one, you can calculate the other. A larger k means faster elimination and a shorter half-life. For example, if k = 0.0866 per hour, then t½ ≈ 8 hours. Understanding this relationship helps you convert between half-life and elimination rate constant and see how they're inversely related.
What does the 'target fraction remaining' interval represent?
This is a purely theoretical calculation showing the dosing interval (τ) that would leave a specified fraction of the starting amount at the end of the interval. For example, if you set the target to 0.25 (25%), the tool calculates how many hours it would take to reach 25% of the initial level using τ = -ln(f)/k. This is for educational intuition only—it is NOT a clinical dosing recommendation. Understanding this helps you see how interval relates to fraction remaining and why theoretical intervals are for learning, not clinical use.
Can I use this calculator to choose a real dosing schedule?
No, absolutely not. This tool is for educational purposes only and uses a highly simplified model. Real drug dosing requires consideration of many factors including drug formulation, absorption characteristics, multi-compartment distribution, patient-specific factors (age, weight, renal/hepatic function), drug interactions, therapeutic windows, and clinical judgment. Always consult qualified healthcare professionals and use validated pharmacokinetic software for clinical decisions. Understanding this limitation helps you use the tool for learning while recognizing that clinical dosing requires validated procedures and professional judgment.
Why is this based on a one-compartment, first-order model?
The one-compartment, first-order model is the simplest pharmacokinetic model and serves as a foundation for understanding drug elimination. It assumes the body acts as a single, well-mixed compartment and that elimination is proportional to the amount present. While many real drugs follow more complex multi-compartment kinetics, this simple model provides useful intuition about half-life concepts and is commonly taught in introductory pharmacology courses. Understanding this helps you see why simplified models are used for education and when more complex models are needed.
What does estimating k from two time-concentration points assume?
When you provide two time-concentration data points, the tool assumes both measurements were taken during the elimination phase (after absorption and distribution are complete). It fits a simple log-linear decline (ln C vs time) to estimate the slope, which gives -k. If the points include absorption/distribution phases, or if concentrations increased rather than decreased, the estimate will be invalid. Understanding this helps you recognize when time-concentration points are appropriate for k estimation and why phase recognition is essential.
How long does it take to reach 90%, 95%, or 99% elimination?
In a simple first-order model: 90% elimination (10% remaining) takes about 3.3 half-lives; 95% elimination (5% remaining) takes about 4.3 half-lives; 99% elimination (1% remaining) takes about 6.6 half-lives. These are commonly cited milestones in pharmacokinetics education and drug washout period estimation. Understanding this helps you estimate elimination times for specific percentages and see why these milestones are useful for drug washout planning.
What is the relationship between dosing interval and drug accumulation?
If dosing intervals are short relative to half-life, drug levels can accumulate because less drug is eliminated between doses. If intervals are long compared to half-life, levels drop significantly between doses. The degree of accumulation depends on the fraction remaining after each interval: if fraction remaining is high (e.g., 0.9), accumulation is significant; if low (e.g., 0.1), accumulation is minimal. Understanding this helps you see how interval selection affects drug accumulation and why proper interval selection is important.
How accurate is the one-compartment model for real drugs?
The one-compartment model is a simplification that works well for educational purposes and some drugs that distribute rapidly and uniformly. However, many real drugs follow multi-compartment kinetics with distinct distribution and elimination phases. The model also ignores absorption, saturable clearance, protein binding, and patient variability. For accurate pharmacokinetic analysis of real drugs, multi-compartment models and patient-specific factors are typically needed. Understanding this helps you see when simplified models are appropriate and when more complex models are required.
Can I use this tool for therapeutic drug monitoring (TDM)?
No. This tool is strictly for educational purposes and does not provide therapeutic drug monitoring recommendations. TDM requires validated assays, patient-specific factors, therapeutic windows, clinical judgment, and integration with patient care. This tool uses simplified calculations that don't account for the many factors required in clinical TDM. Always consult qualified healthcare professionals and use validated TDM protocols for actual patient care. Understanding this limitation helps you use the tool for learning while recognizing that TDM requires validated procedures and clinical expertise.
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