Drug Half-Life Calculator

The Drug Half-Life Formula Explained

The elimination of most medications follows First-Order Kinetics. This means that the body eliminates a constant percentage of the active drug per unit of time, rather than a constant absolute amount (in milligrams) per hour. This is a key pharmacology concept. To model this exponential decay mathematically, we first calculate the elimination rate constant (k):

where t½ is the drug's elimination half-life. Once the rate constant is established, the remaining amount of the drug at any time (t) is determined by the exponential decay formula (the fraction remaining formula):

Or equivalently, expressed in terms of the number of half-lives elapsed:

Note on Zero-Order Kinetics: While most drugs follow first-order decay, a small number of drugs (e.g. alcohol, and aspirin in high doses) exhibit Zero-Order Kinetics. Under zero-order clearance, the body eliminates a constant amount of the drug per unit of time (e.g., exactly 10 grams of alcohol per hour) rather than a percentage, regardless of how much is in the bloodstream. This makes their clearance rates much harder to predict and increases the risk of toxicity.

Common Drug Half-Lives Reference Table

Below is a quick-reference table showing the half-lives of several common medications. We include only these for conciseness — readers can use the search function in the calculator above to access the full database of over 100 medications:

Sources: NIH StatPearls; Merck Manual; FDA Drug Information

Note: For the full 100+ drug database, use the search function in the calculator above.

Drug Detection Windows — What Drug Tests Actually Measure

While drug half-life measures the active drug in the bloodstream, drug tests typically measure metabolites (chemical byproducts) in urine, blood, hair, or saliva. These metabolites can linger in the body far longer than the active drug. For example, Prozac's active metabolite norfluoxetine has a 4–15 day half-life compared to Prozac's 2–7 day half-life, extending the timeline for detection.

Below is a compact drug detection window table:

Sources: SAMHSA; Moeller KE et al., Mayo Clinic Proceedings, 2008

Disclaimer: Detection windows vary by dose, frequency of use, and individual metabolism. These are general estimates only.

Reaching Steady-State — For Patients on Daily Medications

Steady-state is the point at which the rate of drug intake equals the rate of drug elimination. In clinical pharmacology, this state is reached after approximately 4–5 half-lives of regular dosing.

For example, Lexapro (escitalopram) has an average half-life of 30 hours, reaching steady-state in approximately 5–7 days of daily dosing.

Understanding steady-state is critical: daily-dosed drugs like antidepressants, blood pressure medications, and birth control all take time to build up to therapeutic levels. Patients should not expect an immediate physiological effect upon starting their medication course.

The pharmacokinetic formula for steady-state average concentration is:

where F is bioavailability, τ (tau) is the dosing interval, and CL is the clearance rate. (As a simplified explanation: the calculator above models the accumulation curve of this buildup phase).

How Long Do Common Drugs Stay in Your System?

While the calculator above allows you to estimate clearance rates for custom values and specific compounds, it is helpful to look at real-world benchmarks. Below is a detailed look at six of the most common substances, their pharmacokinetics, and clinical detection guidelines: