The first-order absorption rate constant is the rate constant that determines the rate of drug absorption from the site of administration. Larger absorption rate constant results in faster rate of drug absorption.

The absorption rate constant determined after drug administration is an operative rate constant that accounts for all the necessary steps required for drug absorption into the systemic circulation including disintegration, dissolution and absorption.

The first-order absorption rate constant has units of time^-1.

The semilog graph paper allows plotting the data at logarithmic intervals without doing the actual logarithmic transformation of the numbers.

In order to determine the slope of a line drawn on a semilog graph paper it is necessary to do logarithmic transformation for the y values. The slope of the line in this case will be determined from the following relationship:

A semilog graph paper is a graph paper in which the x-axis is a normal linear scale, but the y-axis is a logarithmic scale. Semilog graph paper allows plotting the data at logarithmic intervals without the need for logarithmic transformation.

The y-axis is divided into cycles, each represents 10-fold increase in values (a single log₁₀ unit).

The first step is to plot the plasma concentration-time data and calculate the elimination rate constant (k). A table can be constructed to calculate all the necessary information to calculate the fraction of the drug to be absorbed.

Example: The following plasma cone time data were obtained after a single oral dose of a drug:

Graphically k = 0.33 hr^-1

Is the ratio of the amount of the drug absorbed up to time t to the total amount of the drug that will eventually be absorbed.

Fraction of the drug absorbed =