Half-life (t½) or half-time is defined as the time period required for the concentration of drug to decrease by one-half.

**Zero-Order Half-Life**

**Half-life (t _{½}) **or

Equation 8.9 shows that the t_{½} of a
zero-order process is not constant but proportional to the initial
concentration of drug C_{o} and inversely proportional to the
zero-order rate constant K_{o}. Since the zero-order t _{½}
changes with the decline in drug concentration, it is of little practical
importance. Zero-order equations do not require logarithmic transformations.

Examples of zero-order processes are –

1. Metabolism/protein-drug
binding/enzyme or carrier-mediated transport under saturated conditions. The rate
of metabolism, binding or transport of drug remains constant as long as its
concentration is in excess of saturating concentration.

2. Administration of a drug as a
constant rate i.v. infusion.

3. Controlled drug delivery such as that from i.m.
implants or osmotic pumps.

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