Opioid receptors are a group of G-protein coupled receptors that have opioids as ligands: a substance that is able to bind to and form a complex with a biomolecule, in order to serve a biological purpose.

The opioid receptors are mainly found in the Central Nervous System (C.N.S.) and the gastro-intestinal tract, and include the delta (D) receptors, the kappa (k) receptors and the mu (μ) receptors. Kratom affects mostly the μ (mu) and D (delta) receptors.

An opioid binds to specific opioid receptors in the central nervous system and elsewhere, and elicits a distinct set of neurological responses, inducing the well known effects of opiates and opioids, pleasurable, analgesic and anaesthetic (pain relief), anti-anxiety, antitussive effects, as well as other adverse effects such as vomiting, constipation and the physical addiction and withdrawal issues associated with cessation of their use.