What is the difference between biological half life or plasma half life for a drug ?

The biological half-life of a drug is an estimate of the time it takes for the concentration or amount in the body of that drug to be reduced by exactly one half (50%). It may also be called the elimination half life and it is usually the half-life cited by most drug information sources.

The plasma half-life of a is an estimate of the time it takes for the concentration or amount of that drug in the plasma to be reduced by exactly one half (50%). This can be different from the biological or elimination half-life of a drug because it depends on how well the drug is distributed in the body, if it binds to proteins, does it reach a saturation point, and many other factors. The relationship between biological half-life and plasma half-life can be complex.

Usually only the biological half life (also known as the elimination half-life) is listed, and this is usually given as a range, because it varies so much from person to person.

See Drug Half-Life Explained for more information.