5'-Guanidinonaltrindole (5'-GNTI) is an opioid antagonist used in scientific research which is highly selective for the ú opioid receptor. It is more potent and selective than the commonly used ú-opioid antagonist norbinaltorphimine. It has a slow onset and long duration of action, and produces antidepressant effects in animal studies. It also increases allodynia by interfering with the action of the ú-opioid peptide dynorphin.
In addition to activity at the KOR, 5'-GNTI has been found to act as a positive allosteric modulator of the ñ<sub>1A</sub>-adrenergic receptor (EC<sub>50</sub>âÂÂ=âÂÂ41 nM), and this may contribute to its "severe transient effects".