Question:

Dear Pharmacists,

I have a question with regards to the meaning of the term “Terminal half life”. What does the term “terminal half life” refer to? What is the difference between the term “half life” and “Terminal half life”?

Answer:

The half life is the time it takes for the plasma concentration or the amount of drug in the body to be reduced by 50%. In the simplest model of drug elimination from the body, a drug declines at a constant rate.

However, many drugs are not eliminated from the body at a constant rate. These drugs leave the body in a more complicated pattern of decline. Factors such as how widely the drug is distributed throughout the body and how easily the body is able to remove the drug from the plasma influence the rate of elimination. The rate may change over time rather than being constant. The term “terminal half life” is used for the slowest process of concentraton decrease of a drug from the body.

Submitted by Lisa McDonald
PharmD Candidate, The Ohio State University College of Pharmacy

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