How to Use the Drug Half-Life Calculator
Our drug half-life calculator helps you understand how long a medication remains active in your body. Enter the initial dose and the drug's half-life to see the elimination schedule over multiple cycles.
- Enter the initial dose — the amount of drug administered. Choose your preferred unit: mg (milligrams), μg (micrograms), or g (grams). For weight-based dosing (common in pediatrics and oncology), enable the Weight-based dosing option and enter the dose per kg or lb of body weight.
- Enter the half-life — the time it takes for the drug concentration to reduce by 50%. Select the appropriate time unit: minutes, hours, or days.
- Set the number of cycles — choose how many half-life cycles to display in the results table (5–20).
- Click Calculate to see the full elimination schedule.
What Is the Half-Life of a Drug?
In pharmacology, the half-life (t½) of a drug is the time required for the concentration of the drug in the bloodstream (or the total amount in the body) to decrease by one-half. After each half-life period, only 50% of the previous amount remains.
The half-life helps clinicians determine:
- How often a drug should be administered to maintain a therapeutic concentration
- How long it takes for a drug to be "cleared" from the body (typically 5–7 half-lives)
- Whether drug interactions or toxicity are possible based on remaining concentration
How to Calculate the Half-Life of a Drug
The elimination follows first-order kinetics. The amount remaining after n half-lives is:
Amount(n) = Initial Dose × (0.5)ⁿ
Concentration(n) = (0.5)ⁿ × 100%
Where n is the number of elapsed half-life periods. The time elapsed equals n × t½.
Half-Life of Medication — Examples
| Drug | Half-life | Notes |
|---|---|---|
| Aspirin (acetylsalicylic acid) | 15–20 min | Very short; active metabolite salicylate: 2–3 h |
| Ibuprofen | 1.8–2 h | Short-acting NSAID |
| Amoxicillin | 1–1.5 h | Penicillin antibiotic |
| Metformin | 4–8.7 h | Type 2 diabetes medication |
| Atorvastatin (Lipitor) | 14 h | Statin for cholesterol |
| Amlodipine | 30–50 h | Calcium channel blocker |
| Fluoxetine (Prozac) | 1–4 days | SSRI antidepressant; active metabolite: 4–16 days |
| Diazepam (Valium) | 20–100 h | Benzodiazepine; active metabolite: 36–200 h |
| Levothyroxine | 6–7 days | Thyroid hormone replacement |
| Digoxin | 36–48 h | Cardiac glycoside; narrow therapeutic index |
Frequently Asked Questions
How many half-lives until a drug is fully eliminated?
After 5 half-lives, approximately 96.9% of the drug has been eliminated (only 3.1% remains). After 7 half-lives, about 99.2% is eliminated. Most drugs are considered effectively cleared after 5–7 half-lives.
Does half-life change with dose?
For drugs with first-order kinetics (most medications), the half-life is independent of the dose — the same fraction is eliminated each period regardless of how much was taken. However, some drugs (e.g., phenytoin, alcohol) follow zero-order kinetics at high doses, where a fixed amount (not fraction) is eliminated per unit time.
What affects a drug's half-life?
Several factors can alter the effective half-life of a drug in a specific patient:
- Liver function — most drugs are metabolized hepatically; liver disease prolongs half-life
- Kidney function — renally excreted drugs accumulate in kidney disease
- Age — elderly patients often have reduced clearance
- Drug interactions — enzyme inducers/inhibitors can accelerate or slow metabolism
- Body composition — highly lipophilic drugs may have longer half-lives in patients with higher body fat
What is the difference between half-life and duration of action?
The half-life is a pharmacokinetic property — it describes how quickly the drug is eliminated. The duration of action is a pharmacodynamic property — it describes how long the drug produces its effect. These can differ significantly: a drug may produce effects even at very low (sub-threshold) concentrations, or may have delayed effects due to receptor binding.