When a drug enters the bloodstream there is always a portion that is carried through the liver, which is the principal site of drug metabolism. Some drugs are pharmacologically active to start with, and are metabolized in the liver to form inactive substances before they are eliminated from the body (synthetic reactions).
Some antibacterial drugs are excreted in a partially inactive form, which means they are of no value in treating urinary tract infections. Other drugs may be inactive to start with, but metabolism in the liver results in active substances being released (non-synthetic reactions). For example, chloral hydrate is broken down after absorption to trichloroethanol, which has a sedative action. Excretion of drugs is carried out mainly by the kidney, from which drugs then pass out in the urine. Any kidney dysfunction may reduce excretion; thus lower doses of drugs may be needed to avoid excessive blood levels. Some drugs are also excreted in bile produced by the liver, but can be reabsorbed to some extent from the intestine, which gives them an extended life.
A small contribution to the excretion of drugs is made by the lungs, sweat glands, saliva and breast milk. Because drugs may be excreted in breast milk, it is advisable not to take drugs during lactation.