Does First Pass Metabolism affect Intravenous (IV) medication? Does IV medication completely bypass the liver?

No first pass metabolism does not affect medications given by the Intravenous(IV) route. Yes IV medication does completely bypass the liver.

First pass metabolism means the metabolism of the drug that takes place before the administered drug reaches the systemic circulation from the place it was administered. This is applicable specially when the drug is given orally.

First pass metabolism takes place in the liver(mainly*). As the drug that is taken orally gets absorbed, it is absorbed 1st into the portal venous system. It travels to the liver where this metabolism causes at least some portion of the absorbed drug to be changed into a metabolized form, which is usually an inert one. So only a certain portion reaches systemic circulation. This metabolized portion is different in different drugs. For Eg. Morphine, Diazepam are some drugs with very high First pass effect, so most of the orally taken drugs are metabolized and hence are of no use given orally.

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As IV route means the drug enters systemic circulation directly, no drug enters the liver. So there is no first pass effect in the IV route drug medication. Other routes without first pass effect: sublingual, transdermal, intramuscular, suppositories, inhalational.

*Other causes of 1st pass are also present but most important one is the hepatic enzymes