First Pass Effects/ First Pass Metabolism:

• It is defined as the loss of drug through biotransformation before it enters systemic circulation. It may occur through intestine or liver.

a. Intestinal-first pass effect:

• In this type, drug is metabolized in the GI tract by enzymes present either in gut mucosa or lumen before they are absorbed.
• Microorganism in GI tract also inactivate some drugs.
• Such drugs are not suitable by oral administration due to poor bioavailability.

b. Hepatic first-pass effect:

• In this type, drugs are suitably absorbed across the GI tract and enter portal circulation.
• But, they are rapidly and significantly metabolized during the first passage through liver.
• Ex; nitroglycerine, propranolol