First pass metabolism (also known as the first
pass effect) is when the concentration of certain pharmaceuticals become
reduced before they can be adequately circulated throughout the entire
system. The "first pass" generally refers to the first trip
ingested pharmaceuticals take through the liver, where afterward only a limited
amount of the drug is passed on to the rest of the system. First pass
metabolism is generally applied to oral pharmaceuticals, or ingested
After certain pharmaceuticals are ingested by the body, they are absorbed by
the digestive system after being initially metabolized through digestion.
Afterward, the absorbed pharmaceuticals enter the hepatic portal system, where
they are carried by the portal vein to the liver. In the liver, ingested
drugs under metabolism by enzymes that regulate their bioavailabilty (their
biological accessibility). If first pass metabolism occurs, it means that
the liver will not allow enough of the pharmaceuticals to pass into the blood
stream to be effective at helping the body.
First pass metabolism can be a standard reaction of the average liver to
certain pharmaceuticals or it may the actions of a very particular liver to
either very particular pharmaceuticals or entire assortments of them. In
other words, it may be a defect of the drug or a defect of the patient. When it
is a defect of the drug, the pharmaceutical company has many options to take to
fix the problem. If they are intent of keeping the drug as an ingested
treatment, then they may try to increase the concentration of the drug in the
treatment. Many ingested drugs are usually combined with other innocuous
compounds designed to facilitate passage through the digestive system and the
wall of the gut. However, this can be risky tract as overabundance of the
drug can impair metabolic function and lead to harmful quantities of particular
pharmaceuticals being sent into the system.
More commonly, when a pharmaceutical company has a drug highly susceptible to
first pass metabolism, they will often try to find other means of administering
the drug to the system that bypass the digestive system and the liver
altogether. Most of these processes involve trying to send the drug
directly into the blood stream. By doing this, the drug avoids liver
metabolism and arrives in the blood stream, fully concentrated, to be carried
through the system. One example is sublingual administration.
Sublingual, meaning "under the tongue" in Latin, is where a drug is
taken in the mouth but not swallow. Instead, it is placed under the
tongue where it is absorbed by the thin tissues covering the many blood vessels
under the tongue and sent into the blood stream.
Inhalants are another common form of pharmaceutical administration that
bypasses first pass metabolism. Oral inhalants send the medication
directly to the longs, where it becomes administered to the blood stream
through the process of respiration. With nasal inhalants, the drug is
absorbed by the mucus membranes of the nose and into the blood vessels just
below their surface. The last two means of administration are
injection. Intravenous injection sends the medication directly to the
blood stream through the veins, which will carry it to the heart and allow it
to be pumped then throughout the body. Or intramuscular injection, where
the pharmaceuticals are administered directly to the muscles and absorbed
through the blood stream that way.