Skip to main content

What do you mean by the half life of a drug?

Medically reviewed by Last updated on May 30, 2022.

Official answer


The half-life of a drug is the time taken for the plasma concentration of a drug to reduce to half its original value. Half-life is used to estimate how long it takes for a drug to be removed from your body.

For example: The half-life of Ambien is about 2 hours.

So if you take Ambien after 2 hours the plasma concentration will be reduced to half, after 2 more hours the remaining blood levels will be reduced by another half - so a quarter will be left. Two hours later another half will be removed leaving one eighth and so on. Generally it is considered that it takes 5.5 half-lifes for a drug to be removed from the body, in that it is considered to no longer have a clinical effect.

So for Ambien it would take approximately 11 hours (2 hours X 5.5) to be eliminated from your body.

Elimination of a drug varies from person to person due to factors like age, weight, other medications taken, or other medical conditions present, also kidney function, liver function etc. Therefore half-life is used as a guide or an estimate of how long it may take for the drug to be removed from the body.

See Also: Drug Half Life Explained

Related medical questions

Drug information

Related support groups

  • Ambien (108 questions, 644 members)