Learn more on how the body absorbs and uses medicine: http://www.merckmanuals.com/home/drugs/administration-and-kinetics-of-drugs/drug-absorption
How rapidly drugs are absorbed into the body depends on factors such as the speed of the gastrointestinal tract, how acidic it is, and whether the drug is designed to be absorbed slowly (called slow- or sustained-release preparations). Drugs come in tablets, capsules, skin (transdermal) patches, suppositories, and liquids (solutions). Drugs are transformed (metabolized) in the body by enzymes such as those in the liver or kidneys. Drugs that are inactive when taken, but become active after being metabolized, are called pro-drugs. Drugs are eliminated from the body in the urine (for example, if they are soluble in water or after being metabolized) or feces.
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First published in 1899 as a small reference book for physicians and pharmacists, The Merck Manual grew in size and scope to become one of the world's most widely used comprehensive medical resources for professionals and consumers. As The Manual evolved, it continually expanded the reach and depth of its offerings to reflect the mission of providing the best medical information to a wide cross-section of users, including medical professionals and students, veterinarians and veterinary students, and consumers.