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This calculator provides pharmacokinetic estimates based on first-order elimination. Actual drug levels may vary due to individual patient factors, drug interactions, and organ function.
Calculate drug elimination, time to steady state, and remaining drug levels
Same unit as initial dose/level
| Drug | Half-Life | Steady State | Notes |
|---|---|---|---|
| Amoxicillin | 1-1.5 hours | ~6 hours | Short t½, TID dosing |
| Metformin | 4-8.7 hours | 24-48 hours | BID-TID dosing |
| Lisinopril | 12 hours | 2-3 days | Daily dosing |
| Amlodipine | 30-50 hours | 7-8 days | Long t½, daily dosing |
| Warfarin | 20-60 hours | 5-7 days | Highly variable |
| Fluoxetine | 1-3 days (parent) | 2-4 weeks | Active metabolite 4-16 days |
| Amiodarone | 40-55 days | 4-5 months | Extremely long t½ |
C(t) = C₀ × (0.5)^(t/t½)
Where C₀ is initial concentration/dose, t is time elapsed, and t½ is half-life
ke = 0.693 / t½
The elimination rate constant (ke) describes the fraction eliminated per unit time
t(ss) ≈ 4-5 × t½
After 4-5 half-lives with consistent dosing, drug accumulation reaches 94-97% of steady state
Half-life (t½) is the time required for the plasma concentration of a drug to decrease by 50%. It's a key parameter for determining dosing frequency and predicting when a drug will be eliminated from the body.
With repeated dosing, drug accumulates until the rate of elimination equals the rate of administration. After each half-life, you get closer to steady state: 50% after 1 t½, 75% after 2, 87.5% after 3, 93.75% after 4, and 96.88% after 5 half-lives.
Organ dysfunction can significantly prolong half-life. Renally eliminated drugs have longer half-lives in kidney disease; hepatically metabolized drugs are prolonged in liver disease. Dose adjustments are often needed.
Long half-life drugs (like amiodarone) take longer to reach steady state and longer to be eliminated. This affects loading dose requirements, time to see full effect, and duration of side effects after discontinuation.
After stopping a drug, it takes about 4-5 half-lives to eliminate ~97% of the drug. For drugs with short half-lives, withdrawal effects may appear quickly. For long half-life drugs, effects may persist for days to weeks.
Yes, half-life can vary significantly based on age, weight, organ function, genetic polymorphisms, drug interactions, and disease states. Published half-life values are averages from study populations.