Drugs affect only the rate at which existing biologic functions
proceed. Drugs do not change the basic nature of these functions
or create new functions. For example, drugs can speed up or slow
down the biochemical reactions that cause muscles to contract,
kidney cells to regulate the volume of water and salts retained
or eliminated by the body, glands to secrete substances (such as
mucus, stomach acid, or insulin), and nerves to transmit
messages. Drugs cannot restore structures or functions already
damaged beyond repair by the body. This fundamental limitation
of drug action underlies much of the current frustration in
trying to treat tissue-destroying or degenerative diseases such
as heart failure, arthritis, muscular dystrophy, multiple
sclerosis, and Alzheimer's disease. Nonetheless, some drugs can
help the body repair itself. For example, by stopping an
infection, antibiotics can allow the body to repair damage
caused by the infection. Some drugs are hormones, such as
insulin, thyroid hormones, estrogen