When a doctor prescribes a drug to a patient, there are several vital decisions the doctor needs to make prior to the prescription. Some of these decisions would be regarding the patients age, existing disease conditions, severity of the illness, side effects of the drugs, behavior of a drug after absorption...etc. All these decisions would be aimed at knowing what would happen to the drug once taken by the patient as well as how effective the drug would be in relieving the intended illness. Among the many things a doctor considers before prescribing would be the state of the patient's liver.
Why consideration does need to be given to the liver?
The reason for this consideration is because, the liver will act on the drugs which are taken orally and inflicts changes on these substances either making them more effective or less effective or even dangerous.
Due to the fact that a drug entering through the mouth will encounter its metabolism in the first organ it goes through, the process is widely known in the medical field as 'first pass metabolism' and the place it happens is none other than in the liver. It would be right to state that the characteristic feature of this metabolism is it prevents a drug from entering the systemic circulation without undergoing the process of 'first pass metabolism'.
Let us understand how this process takes place.
When a person takes medications orally, it will travel down the esophagus into the stomach and thereafter into the small intestines. Various factors in the gut will now act upon the medications and digest them into absorbable elements. The factors that contribute in this regard would be the liver enzymes, the bacterial enzymes in the gut as well as the enzymes from other glands.
Following digestion, the gut wall will absorb these elements and via the portal vein would allow it to pass through to the liver. As this happen, only a small portion of the drug would be absorbed and the rest would be expelled.
Once the breakdown products of the drugs enter the liver and its cells, it will undergo further breakdown and metabolism which is what we call the 'first pass metabolism'. In most instances, this would result in formation of 'active elements' of the drug which ultimately would be released into the circulation.
What we can understand from this phenomenon is that, when given orally, only fraction of a drug which is altered in many different ways would reach the systemic circulation.
Why does 'first pass metabolism' related only to oral medications?
This is because, when other routes of entry are used, such as suppository, sublingual, intravenous...etc; the drugs will be directly introduced into the circulation thus bypassing the 'liver metabolism' or else the 'first pass metabolism'. Therefore, these routes will make available a higher concentration of drugs into the circulation than it does through the oral route.