Determination of the Absorption Rate Constant

b- Wagner-Nelson Method

This method uses the calculated fraction of the administered dose remaining to be absorbed at different time to determine the absorption rate constant.

After oral drug
administration:

Drug in the GIT

Drug absorption

Drugs in all parts of the body

Drug elimination

Drug eliminated

At any time:

The amount of drug absorbed = amount of drug in the body (at time t) + amount of drug eliminated (up to time t)

At time infinity:

The amount of drug absorbed = 0 + amount of drug eliminated (up to time infinity)

Fraction of dose absorbed at any time = amount of drug in the body + amount of drug eliminated ∣ t= t t=0

amount of drug eliminated ∣ t= ∞ t=0

Fraction of dose remaining to be absorbed (at any time) = 1 - fraction of dose absorbed (up to this time)

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