What is the “First-Pass Effect”, and what types of tablet are intended to avoid it?

The "First-Pass Effect" or "First-Pass Metabolism" refers to the phenomenon where the concentration of a drug is significantly reduced before it reaches the systemic circulation. It is the fraction of lost drug during the process of absorption which is generally related to the liver and gut wall.

Here's how it works, step by step:

1. After oral administration, the drug is absorbed by the digestive system and enters the hepatic portal system.
2. It is carried through the portal vein into the liver before it reaches the rest of the body.
3. The liver metabolizes many drugs, sometimes to such an extent that only a small amount of active drug emerges from the liver to the rest of the circulatory system.
4. This first pass through the liver thus may greatly reduce the bioavailability of the drug.

To avoid the first-pass effect, certain types of tablets or drug delivery systems are designed. These include:

1. Sublingual/Buccal tablets: These tablets are placed under the tongue or between the gum and cheek, where they dissolve and the drug is absorbed directly into the systemic circulation, bypassing the liver.

2. Transdermal patches: These are adhesive patches applied to the skin which deliver a specific dose of medication through the skin and into the bloodstream, again bypassing the liver.

3. Rectal suppositories: These are inserted into the rectum where the drug is absorbed by the rectal tissue and enters the systemic circulation, bypassing the liver.

4. Injectable/Intravenous drugs: These are directly introduced into the systemic circulation, completely bypassing the liver.

These methods are used to increase the bioavailability of drugs and ensure that they can exert their therapeutic effects.