Oral route of administration, per os

VIA ORAL, also called PER OS, from the Latin per, which means through, and from the Latin òs, òris, which means mouth (OS is also an acronym for Oral Somministration, which means Oral Administration).

Administration routes

  • enteral
    • Oral
    • Sublingual
    • Rectal
  • parenteral
    • Intravenous
    • Intramuscular
    • subcutaneous


Through oral administration, only a minimal part of the drug reaches absorption and the site of action. Citing the example of a tablet, the latter will undergo significant disintegrations starting from the mouth, and then continue into the stomach and intestine. At this point what remains of the drug will be absorbed and transported to the liver, where it will undergo metabolism due to the first hepatic passage. At the end of all these disintegrations and metabolizations, the drug will be distributed in the body. The set of all these phenomena determines the bioavailability of the drug within our body.

The bioavailability of the drug is the fraction of undegraded drug that reaches the systemic circulation and is able to distribute itself throughout the body.

Bringing a comparison between the oral and intravenous route of administration, the first has a clearly lower bioavailability than the second, because the drug undergoes considerable modifications before reaching the bloodstream. This does not happen if the drug is injected directly into the bloodstream (intravenously).

The bioavailability of an orally administered drug depends on various factors, such as the functional state of the digestive tract, the rate of transit of the drug in the stomach and intestines, the activity of the bacterial flora, the composition of the drug and finally the possible metabolism of the drug. In addition, factors such as gastric pH (hypochloridria or hyperchlorhydria), dissociation of the drug, its oil / water breakdown and the blood supply to the absorption area can affect the bioavailability and absorption of the drug. Foods also play a very important role because they can increase or decrease the rate of drug elimination. In the case of increased elimination there is a rapid transit of the drug in the gastro-intestinal tract, with a consequent reduction in absorption and minor pharmacological effects; in the presence of a reduced elimination there is a prolonged permanence of the medicine in our body, with a prolonged therapeutic effect over time, but with the possibility of more or less serious side effects.


Easy execution


Not very risky

Absorption modulation


Inactivations by enzymatic hydrolysis (insulin and protein substances)

Inactivations due to gastric acidity (penicillin G)

First hepatic passage and intestinal flora destruction

Other advantages of oral administration:

  • it is the most natural way there is.
  • Allows greater patient participation in the care plan.
  • It does not require training, only information.

Further disadvantages of oral administration:

  • It may no longer be pleasing to the patient.
  • Ineffective in the presence of diseases of the gastro-intestinal tract.
  • Impractical in dysphagic patients.
  • Not usable in the last hours of life.
  • It requires good patient cooperation.
  • It cannot be used for drugs that are destroyed by gastric juices, that form with non-absorbable complex foods or that are extensively metabolized by the liver before reaching the general circulation.
  • Since absorption can be irregular, there is little control of the dosage.
  • Since absorption is slow it cannot be used in emergency therapies.

We remind you that if a drug is absorbed in the oral cavity or at the lower and middle hemorrhoidal plexus, the first hepatic passage is avoided, thus further modifications to the drug. If, on the other hand, the drug is absorbed at the level of the upper hemorrhoidal plexus and at the gastro-intestinal level, it undergoes the first hepatic passage, therefore, before entering the circulation, the medicinal product undergoes modifications.


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