Drug Half-Life Calculator
Calculate the elimination half-life of a drug and determine how long it takes for a medication to be cleared from the body. Useful for pharmacokinetic analysis and medication dosing.
Results: -
| Remaining Amount | - |
|---|---|
| Eliminated Amount | - |
| Time to 90% Elimination | - |
| Time to 95% Elimination | - |
| Time to 99% Elimination | - |
Elimination Over Time
| Half-Lives | Time | Remaining | Eliminated |
|---|
About Drug Half-Life Calculator
The half-life of a drug is the time it takes for the concentration of the drug in the body to be reduced by half. This is an important pharmacokinetic parameter that helps determine dosing intervals and how long a drug remains in the body. Understanding drug half-life is crucial for proper medication management and avoiding toxicity.
How Drug Half-Life Works
Drug elimination typically follows first-order kinetics, meaning a constant fraction of the drug is eliminated per unit time. The half-life (t½) is related to the elimination rate constant (k) by the equation:
t½ = ln(2) / k ≈ 0.693 / k
Where ln(2) is the natural logarithm of 2 (approximately 0.693). The remaining amount of drug after time t can be calculated using:
A = A0 × (1/2)t/t½
Where A is the remaining amount, A0 is the initial amount, and t is the elapsed time.
Key Concepts
Steady State
When a drug is administered repeatedly at regular intervals, it reaches a steady state concentration in about 4-5 half-lives. At steady state, the amount administered equals the amount eliminated during each dosing interval.
Time to Elimination
It takes approximately:
- 3.3 half-lives to eliminate 90% of a drug
- 4.3 half-lives to eliminate 95% of a drug
- 6.6 half-lives to eliminate 99% of a drug
Factors Affecting Half-Life
Several factors can influence a drug's half-life:
- Age: Half-life may be longer in elderly patients
- Liver function: Affects metabolism of many drugs
- Kidney function: Impacts elimination of water-soluble drugs
- Body composition: Fat-soluble drugs may have longer half-lives in obese patients
- Drug interactions: Can inhibit or induce metabolic enzymes
Clinical Applications
Understanding drug half-life is essential for:
- Determining appropriate dosing intervals
- Predicting time to reach steady state
- Estimating how long a drug will remain in the body after discontinuation
- Adjusting doses in patients with impaired liver or kidney function
- Managing drug interactions
Limitations
While half-life is a useful parameter, it has some limitations:
- May vary between individuals due to genetic and physiological differences
- Can change with disease states or organ dysfunction
- Doesn't account for non-linear pharmacokinetics seen with some drugs
- May not reflect duration of pharmacological effect for drugs with active metabolites