I am a care-giver and not a pharmacologist, but according to the Merck
Manual (15th Ed.), the biologic half-life is a convenient parameter. It
is the time required for the plasma drug concentration or the amount in
the body to decrease to 50%. For most drugs, the half-life remains
constant regardless of how much drug is in the body. It is related to
the elimination rate constant by:
Half-life = 0.693/Elimination rate constant
but the elimination rate constant is a function of how a drug is cleared
from the blood by the eliminating organs and how the drug distributes
throughout the body.
Pharmacology is a very complex science when it involves simple drugs,
but when the rate constant is other than a simple first degree order,
this becomes far more complex when dealing with such a drug as L-dopa
that becomes efficacious only upon reaching the brain. This is a
biological term and is usually finite.
Then there is radioactive half-life which is the time it takes for a
number of a nuclide atoms to decay to half the original number of
atoms. This decay varies from seconds to thousands of years and is
asymptotic.
