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

A zero-order process is a process that has a constant rate. The rate of the zero-order process does not depend on the amount of the reactant involved in the process.

If the absorption of a drug follows zero-order kinetics, this means that there is a constant amount of the drug absorbed per unit time, regardless of the administered dose.

Because the rate of drug absorption is constant (zero-order absorption) the fraction of the dose absorbed per unit time after administration of a large dose, is much smaller than the fraction of the dose absorbed per unit time time after administration of smaller doses.

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.

A first-order process is a process that has a rate proportional to the amount of the reactant involved in this process.

For example, if the dg elimination rate is proportional to the amount of the drug in the body, this means that this drug is eliminated by a first-order process. The proportionality constant is the first-order elimination rate constant. i.e.

Elimination rate = elimination rate constant x drug amount

The rate of drug elimination decreases with time because the amount of the drug remaining decreases with time.

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-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.

A rectangular graph paper is a graph paper where the x-axis and the y-axis are divided into equal divisions. Each of these divisions represents a fixed value throughout the x or the y axes (linear scale).

The x-axis and the y-axis may cover different ranges of the graphed variables. The minimum value on the axes may or may not be zero.

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: