Half-Life in Pharmacology

Instructor: Ian Lord

Ian has an MBA and is a real estate investor, former health professions educator, and Air Force veteran.

In pharmacology, the concepts of half-lives and steady states are relevant to a patient. In this lesson we will define what a half-life is in pharmacological terms and explain how it is relevant.


Sarah has taken an interest in understanding more about how her prescription medications work. In reading the literature that comes with each prescription, she has seen a time mentioned labeled as half-life. What is that? In this lesson we will explain to Sarah what the half-life of a medication is and what it means about that medication's effects on her body.

The half-life of a pharmaceutical refers to the amount of time it takes for the concentration of that substance in the body to be reduced by one half. This can be measured by testing for the amount of a medication in Sarah's blood plasma. For example, the medication lamotrigine used for seizures has a half-life of approximately 15-30 hours depending on the individual patient. Clearance refers to the process of the body removing the medication from plasma. This can occur by destroying the medication, passing it out of the body, or transferring it to the intracellular fluid elsewhere in the body.

Steady State

At a certain point the amount of medication coming into and going out of Sarah's body reaches a point of equilibrium. After a medication has been taken for five or six half-lives, a point known as steady state has been reached. So if Sarah's dose of lamotrigine has a half-life of 15-30 hours it will take somewhere between three and ten days to reach steady state levels of the medication in her body. The longer the half-life of a medication, the longer it will take to get to a steady state of concentration in the blood. In some cases a physician might prescribe a larger starting dose followed by a maintenance level dose in order to reach a steady state faster.


So what does half-life and steady state mean on a practical level for Sarah? Many medication side effects occur at times when concentrations in the blood are rising or dropping. Undesirable side effects often go away when a steady state has been achieved. Sometimes a drug has different effects in a steady state compared to when the amount in the body is rising or dropping.

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