Drug Half Life Calculator

Drug Half Life Calculator

Estimate the exponential decay of a drug in the body by just providing the dosage and the half-life of the drug to the calculator.

Half-life (sec):

hrs ▾

Dosage:

mg ▾

Use the drug half-life calculator to determine the amount of a drug still present in a patient’s body when the half-life and dosage are known.

What Does Half-Life Mean in Drugs?

The half-life of a drug is the time required for the concentration of a medication in the body to decrease by 50%. It is commonly represented by the symbol t_1/2.

Patient-Specific Factors That Can Influence Drug Half-Life:

Age of the patient
Efficiency of blood circulation
Dietary habits
Excess body fluids
Gender differences
Kidney performance
Liver health
Body fat or obesity
Smoking habits

How to Calculate the Half-Life of a Drug

The following mathematical expression is used to calculate the half-life of a drug:

Dosage(t) = Dosage(0) × 0.5^{t / T}

Where:

T = half-life of the drug
t = time elapsed since the drug was administered
Dosage(t) = amount of drug remaining in the body at time t
Dosage(0) = initial dose given to the patient

Steps to Calculate Drug Half-Life

Identify the initial drug dosage.
Determine the drug’s half-life duration.
Substitute these values into the half-life formula as shown below.

Example:

Assume the following:

Drug Half-Life = 1 minute
Initial Dosage = 10 g

Calculate the remaining drug amount after one half-life.

Solution:

First, convert grams to milligrams:

Dosage(0) = 10 × 1000 = 10000 mg

Substitute the values into the half-life formula:

Dosage(t) = 10000 mg × 0.5^{(1 / 1)}

Dosage(t) = 5000 mg

Thus, after one half-life, 5000 mg of the drug remains in the patient’s body.

Half-Life of Drugs Chart

Below is a table showing commonly used substances and their respective half-lives.

Generic Name	Brand Name Examples	Half-Life (T_1/2)
Alprazolam	Xanax	6–12 hours
Amiodarone	Pacerone	15–142 days
Amphetamine	Adderall, Dexedrine	10–12 hours
Atenolol	Tenormin	6–7 hours
Clonazepam	Klonopin	18–50 hours
Cocaine	–	50 minutes
Diazepam	Valium	20–100 hours
Donepezil	Aricept	70 hours
Dutasteride	Avodart	5 weeks
Erenumab	Aimovig	28 days
Fluoxetine	Prozac	2–4 days
Lead	–	28–36 days
Mercury	–	65 days
Methamphetamine	Desoxyn	6.4–15 hours
Methylphenidate	Concerta, Ritalin	2–3 hours
Plutonium	–	40 years (liver), 100 years (bone)
Phenytoin	Dilantin, Phenytek	7–42 hours
Tetrahydrocannabinol (cannabis/marijuana)	–	Infrequent users: 1.3 days; Regular users: 13 days

FAQs

Is Half-Life the Same as Drug Duration?

No. Half-life refers to the time required for a drug’s concentration to reduce by half in the body, whereas drug duration describes the overall length of time the drug remains active in the body.

How many half-lives are required to eliminate a medication?

Most drugs take approximately 4–5 half-lives to be cleared from the body. After this period, the drug concentration usually falls below clinically effective levels and is considered eliminated. Increasing the dose can extend the total elimination time.

References:

goodrx.com: What Does Drug Half-Life Mean?