1. Health
Send to a Friend via Email

Discuss in my forum

Medication Half-Life

By

Updated August 04, 2007

Definition:
A medication's half-life is the time it takes for the plasma concentration of a drug to reach half of its original concentration. More simply put, the half-life of a drug is how long it takes for half of it to be eliminated from the bloodstream.

One impact of half-life is found in the SSRI antidepressants. People taking SSRIs with short half-lives are much more likely to experience SSRI Discontinuation Syndrome. People taking an SSRI with a long half-life such as Prozac need to wait far longer between stopping Prozac and starting to take an MAOI antidepressant (see MAOI Antidepressants).

Examples:
Interestingly, no matter what the half-life of a medication is, it takes about 4 half-life units for the concentration of a medication in the system to reach a steady state. Thus if you begin taking a medication with a half-life of 24 hours, on the fifth day the rate of intake of the drug will approximately equal the rate of elimination. If the half-life is 12 hours, you'll reach that state at the beginning of the third day.
Related Video
How to Choose Pain Medication

©2014 About.com. All rights reserved.

We comply with the HONcode standard
for trustworthy health
information: verify here.