Posted by Donna (63.20.108.1) on November 30, 2001 at 17:18:37:
half-life
1. Time required for half the nuclei of a radioactive substance to lose their activity by undergoing radioactive decay. 2. Time it takes for a radioactive substance to reduce to one-half its energy due to metabolism and excretion. 3. In biology and pharmacology, the time required by the body, tissue, or organ to metabolize or inactivate half the amount of a substance taken in.This is an important consideration in determining the proper amount and frequency of dose of drug to be administered. 4. Time required for radioactivity of material taken in by a living organism to be reduced to half its initial value by a combination of radioactive decay and biological elimination.
"Taber's Cyclopedic Medical Dictionary," Copyright © 2001 by F. A. Davis Co., Phil., PA
After the first half-life, you divide what's left in half, and again what's left until there's none left.
Some meds take a very long time and some don't. Read the insert in the med. box or ask your pharmacist.