Pharmacology Notes

The half-life of a drug is the time it takes for one-half of the drug to be eliminated by the body. Factors that affect a drug’s half-life include its rate of absorption, metabolism, and excretion. Knowing how long a drug remains in the body helps determine how frequently it should be administered.

A drug that’s given only once is eliminated from the body almost completely after four or five half-lives. A drug that’s administered at regular intervals, however, reaches a steady concentration (or steady state) after about four or five half-lives. Steady state occurs when the rate of drug administration equals the rate of drug excretion.