Drug Half-Life Calculator

Understanding Half-Life

The biologic half-life of a drug is the time it takes for the concentration of the substance in your blood to be reduced by exactly one-half (50%).

Drug elimination follows exponential decay kinetics (First-Order Kinetics). This means the body eliminates a constant percentage of the drug per hour, not a constant amount. For example, if you have 100mg of a drug with a 2-hour half-life:

Starts: 100 mg
After 2 hrs: 50 mg (50%)
After 4 hrs: 25 mg (25%)
After 6 hrs: 12.5 mg (12.5%)

The "5 Half-Lives" Rule

In clinical pharmacology, a drug is considered to be effectively eliminated (washed out) from the system after 5 half-lives. By this point, roughly 97% of the original dose has been removed, and the remaining 3% is usually biologically negligible.

Medical Clearance Standard

This "5x Rule" is also used to determine how long it takes to reach a "Steady State" concentration when starting a new daily medication.

Factors Affecting Clearance

Metabolism (Liver)The efficiency of CYP450 enzymes in your liver determines how fast drugs are broken down.

Kidney Function (GFR)Many drugs are excreted in urine. Poor kidney function (low GFR) slows down elimination significantly.

AgeMetabolism slows down with age. Seniors often need lower doses because drugs stay in their system longer.

Body CompositionFat-soluble drugs (like THC) can get trapped in adipose tissue, extending their detection window for weeks.

Frequently Asked Questions

How long does Caffeine stay in your system?

The average half-life of caffeine is roughly 5 hours. If you drink a large coffee (200mg) at 4 PM, you will still have 100mg (equivalent to a shot of espresso) in your blood at 9 PM, which can significantly disrupt deep sleep. It takes roughly 24 hours to fully eliminate caffeine from the body.

Does drinking water flush drugs out faster?

Generally, no. While hydration supports kidney function (renal clearance), most drugs are metabolized by the liver or bound to proteins in the blood. Drinking excess water will not significantly speed up the metabolic half-life of substances like antidepressants, ADHD medication, or antibiotics.

When is a drug considered 'out of your system'?

Clinically, a drug is considered "washed out" after 5 half-lives, at which point roughly 97% of the dose has been eliminated. For example, if a drug has a half-life of 12 hours, it will be effectively gone in 60 hours (2.5 days).

Why do half-lives vary between people?

Genetics play the biggest role. Enzymes in the liver (specifically the CYP450 family) break down drugs. Some people are "Rapid Metabolizers" (clear drugs fast), while others are "Poor Metabolizers" (drugs build up to toxic levels). Age, liver disease, and obesity also alter the clearance rate.

Does half-life determine drug testing detection windows?

Not always. Half-life measures the active drug in the blood. However, drug tests often look for metabolites (leftover byproducts) in urine, which can linger for days or weeks after the active effect has worn off.

References

National Institutes of Health (NIH). StatPearls: Drug Half-Life.
Merck Manual. Drug Excretion and Elimination.
FDA. Drug Safety Communications.
Psychology Today. Understanding Medication Duration.

Tools & Data Verified by the EverydayCalculators Medical Research Team.

Last updated: January 2026.

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