Oral administration (PO) Gastrointestinal absorption
Drugs are often given orally, despite the fact that the absorption after oral administration can be quite variable. Dosage form design may also be used modify the rate of absorption (see drug absorption from intestine). How the drug is absorbed can depend on size and the degree of ionization As the drug travels through the gastrointestinal tract, it moves through different pH environments. Since many drugs are either weak acids or weak bases, this results in a change in the drug's ionization status. Once a drug is absorbed orally, it is transported to the general circulation via the liver. Thus drugs which are extensively metabolized will be metabolized in the liver during absorption, never reaching their full concentration in the systemic circulation. See first-pass
Convenient - safe, no pain, easy to take.
Cheap - no need to sterilize, compact, automated machines produce tablets in large quantities.
Variety of dosage forms available - fast release tablets, capsules, enteric coated, layered tablets, slow release, suspensions, mixtures
Sometimes inefficient - high dose or low solubility drugs may suffer poor availability, only part of the dose may be absorbed, and there is the first-pass effect.
See how absorption effects the oral availability of drugs by clicking here.