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How Oral Drugs Are Metabolized in the Body

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Many prescription drugs taken orally are absorbed in the small intestine. The amount of medication that reaches the bloodstream depends on how much is absorbed through the gastrointestinal tract.  Here’s a brief description of how drugs move through the body … 

Some portion of a drug may be lost as it is digested. Enzymes (proteins that facilitate biochemical reactions) in the gastrointestinal tract break down drugs, as do bacteria that normally live there. Drugs also interact with foods and beverages, in some cases reducing but in other cases increasing the amount that gets absorbed. 

After being absorbed through the gut, drug molecules travel via the portal vein to the liver (the largest gland in the body, weighing 3 pounds on average). Most of the work of drug metabolism is done in the liver by enzymes such as the cytochrome P-450 system. As the liver metabolizes drugs, it may produce chemical byproducts that are toxic. This is why taking too much of certain drugs or taking them too often can harm the liver. The most common example of such a drug is acetaminophen (Tylenol). 

Whatever remains of a drug after the liver metabolizes it enters the hepatic vein, which carries blood from the liver to the heart. The heart then pumps the drug molecules out into the general bloodstream, which carries the drug throughout the body to its eventual target organ(s) -- and to many other locations as well. Any molecules of the medicine that remain after traveling through the circulatory system eventually re-enter the liver via the hepatic artery, where the metabolic system can process them further. 

The body excretes or eliminates water-soluble medications and their breakdown products (metabolites) primarily in the kidneys, and the metabolites then pass out of the body through urination. Some medications or byproducts of drug metabolism that were handled by the liver pass back into the digestive tract through bile and later exit the body in the feces. Medications may also leave the body in saliva, sweat, exhaled air and even a mother's breast milk.


Posted in Prescription Drugs on June 7, 2011

Medical Disclaimer: This information is not intended to substitute for the advice of a physician. Click here for additional information: Health After 50 Disclaimer

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Health After 50 Alerts registered users may post comments and share experiences here at their own discretion. We regret that questions on individual health concerns to the editors cannot be answered in this space.

The views expressed here do not constitute medical advice, and do not represent the position of Scientific American Health After 50 or Remedy Health Media, LLC, which has no responsibility for any comments posted on this site.

So, when a drug causes diarrhea (antibiotics), does that mean none of the drug is getting into the bloodstream?

Posted by: dollfriend | June 7, 2011 12:16 PM

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