What does “First-pass metabolism” mean?

The breakdown of a swallowed drug by the liver before it reaches the general circulation.

First-pass metabolism in plain English

The breakdown of a swallowed drug by the liver before it reaches the general circulation.

It is the main reason oral cannabinoid preparations have low and variable bioavailability, and why onset can take one to three hours.

Preparations absorbed under the tongue or inside the cheek partly bypass this, which gives a faster and somewhat more predictable onset.

Frequently asked questions

What does first-pass metabolism mean?

The breakdown of a swallowed drug by the liver before it reaches the general circulation.

Sources and further reading

Every factual statement on this page is drawn from the following public sources. Links open in a new tab and are not affiliated with Flora House.

  1. NICE guideline NG144: Cannabis-based medicinal products
  2. NHS: Medical cannabis (and cannabis oils)
  3. MHRA: The supply, manufacture, importation and distribution of unlicensed cannabis-based products for medicinal use

Patient guide · Written by the Flora House editorial team · Last reviewed: 8 September 2026