Free Drug Half-Life Calculator

Understanding Drug Half-Life

The drug half-life calculator is a practical medication half-life calculator that helps you determine how much of a drug remains in a patient’s system over time. This pharmacology half-life tool also serves as a drug concentration calculator by showing the proportion of the original dose still present in the body at any point after administration.

Drug elimination half-life refers to the time required for the plasma concentration of a medication to reduce by half. This concept is distinct from the radioactive half-life seen in nuclear decay. If you are interested in radioactive decay principles, several dedicated calculators cover that topic.

How to Use the Calculator

To get started, you need two inputs:

• Half-life of the drug – enter the time (in minutes, hours, or days) it takes for the drug’s concentration to halve. The calculator automatically manages the unit.
• Initial dosage – the amount administered at the start.

The tool then generates a table covering at least five half‑life cycles, showing:

• Time elapsed since administration
• Remaining dosage in the body
• Concentration expressed as a fraction of the original dose

The Formula Behind the Calculation

The drug elimination half‑life calculator uses the exponential decay equation:

$$\text{Dosage}(t) = \text{Dosage}(0) \times 0.5^{(t/T)}$$

where:

• $T$ – half-life of the drug,
• $t$ – time that has passed since the initial dose,
• $\text{Dosage}(0)$ – the original administered amount,
• $\text{Dosage}(t)$ – the amount remaining after time $t$.

Why Half-Life Matters

Knowing a drug’s half-life helps clinicians assess its potency and toxicity over time. With this information, they can design a dosing schedule that maximizes therapeutic effects while minimizing adverse reactions. This is especially important for drugs with a narrow therapeutic window, such as those that can prolong the QT interval and lead to heart arrhythmias. In pharmacology, half-life is essentially the opposite of doubling time.

Half-Life Examples for Common Medications

If you aren’t sure about the half-life of a specific drug, the table below lists elimination half-lives for a selection of common medications. Values can vary based on individual patient factors (age, liver function, etc.).

Medication	Half-Life (range)
Acetaminophen	1–4 h
Adderall	9–14 h
Amiodarone	26–107 days
Carbamazepine	14–47 h
Ciprofloxacin	4 h
Diazepam	21–37 h
Digoxin	36–48 h
Epinephrine	2–3 min
Fentanyl	3–12 h
Fluconazole	20–50 h
Ganciclovir	3–6 h
Haloperidol	14–41 h
Heparin	3–6 h
Ketoconazole	2–8 h
Lidocaine	1–2 h
Methotrexate	3–10 h
Metronidazole	8 h
Morphine	1–7 h
Oxycodone	3–5 h
Phenobarbital	81–117 h
Propranolol	2–6 h
Streptomycin	5–6 h
Tetracycline	7–11 h
Valproic Acid	9–16 h
Warfarin	20–60 h

Epinephrine has one of the shortest half-lives (only 2–3 minutes), while amiodarone can take more than 100 days to eliminate half of a dose.

Important Note

This calculator is designed to be a reliable reference tool. It does not replace professional medical assessment. If you have concerns about drug dosing or elimination, always consult a healthcare provider.