This question has also been asked and answered here: What is the half-life of 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.
A drug's half-life is the amount of time for the concentration of drug in the body to be reduced by half.
A drug's plasma half-life is the amount of time required for the drug to be eliminated from the plasma.
The connection between the two is considering the amount of drug concentration to plasma ratio, which can vary.
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