Opioids

Natural or Synthetic

Chemical Structure Class

Receptor Binding

Morphine

Natural

4,5-epoxy morphinan ring

μ +++

κ +

Codeine

Natural

4,5-epoxy morphinan ring

μ +

Hydromorphone

Semi-synthetic

4,5-epoxy morphinan ring

μ +++

Oxycodone

Semi-synthetic

4,5-epoxy morphinan ring

μ +

κ ++

Tramadol

Synthetic

phenylpiperidine

μ +

Tapentadol

Synthetic

phenylpiperidine

μ +++

Fentanyl

Synthetic

phenylpiperidine

μ +++

Methadone

Synthetic

diphenylheptylamine

μ +++