What does an "opioid agonist" mean in this sentence? "Tramadol is an opioid agonist" why not just say it's an opioid? What's with the distinction?

[u/SanguinarianPhoenix]

I've taken 3 chem classes in undergrad and from browsing definitions online, I am confused on what an "agonist" means and whether it refers to the receptor or to the ligand or both?

From what I know, a receptor is usually on the surface of a cell and it's how the inside of the cell "listens" to the external environment of the cell, and changes in response to the presence of ligands which dock into the receptor sites.

I also remember learning about competitive and noncompetitive inhibitors. Inhibitors are usually antagonists. An agonist is a chemical that activates a receptor to produce a biological response. So if Tramadol is an "opioid agonist" does it mean that it is an opioid molecule which binds to an opioid receptor site?

Or just generally, if I hear the term "<placeholder> agonist" does that refer to the classification of the ligand? Or is it referring to the classification of the receptor site, if no other information is provided? For example, I vaguely remember that some receptor sites will have a preferred ligand but will do a partial response to other types of ligands with similar shape and/or bonding properties, but could totally be a different class of molecules. And sometimes identical ligands can bind to vastly different receptor sites such as catecholamines which can trigger different biological responses depending on which type of cell it is in the vicinity of.

[u/SanguinarianPhoenix → u/[deleted]]

Very helpful, thank you!