Table 1.

Receptor type specific effects of opioid drugs (agonists).

Receptor Type	Main Effects of Receptor Agonism	Receptor Agonist	Receptor Type Selectivity 1	Ref.
µ (mu)	analgesia bradycardia cough suppression euphoria (rush) miosis (pupil constriction) physical dependence reduced gastrointestinal motility (constipation, cramps) respiratory depression (including decrease in sensitivity of respiratory center for CO2) sedation	morphine (reference)	µ/δ: 0.006–0.040 µ/κ: 0.023–0.059	[13,14,15,16]
		codeine (OTC drug)	µ/δ: 0.049–0.051 µ/κ: 0.033–0.044	[13,15]
		dihydrocodeine (OTC drug)	µ/δ: 0.036–0.055 µ/κ: 0.018–0.023	[13,15]
		loperamide (OTC drug)	µ/δ: 0.003	[17]
δ (delta)	cough suppression (disputed) gastrointestinal dysmotility mood modulation respiratory depression spinal analgesia (pain control)	morphine (reference)	δ/µ: 25.000–159.091	[13,14,15,16]
		SNC80 2	δ/µ: 0.002	[18,19]
		BW373U86 3	δ/µ: 0.120	[20]
κ (kappa)	cough suppression dysphoria (profound sensation of dissatisfaction and unease) gastrointestinal dysmotility hallucinations peripheral analgesia physical dependence pupil constriction sedation	morphine (reference)	κ/µ: 16.950–42.636	[13,14,15,16]
		butorphanol 4	κ/µ: 0.545	[21,22]
		pentazocine 4	κ/µ: 0.564–0.772	[14]
		nalorphine 5	κ/µ: 0.667–0.895	[14]

¹ Ratio of Ki-values (lower value = more selective); ² experimental drug (convulsant/antidepressant/anxiolytic); ³ experimental drug (convulsant/antidepressant/analgesic); ⁴ therapeutic analgesic; ⁵ opioid overdose antidote.