Agonist

An agonist is a chemical that binds to a receptor and activates the receptor to produce a biological response.

In terms of cannabis and humans: cannabinoids are agonists that bind to receptors in the human body to produce the experienced effects .

It is important to note that if the human body did not evolve to develop cannabinoid receptors (referred to as CB1 and CB2), then ingesting cannabis would yield no results.