https://en.wikipedia.org/wiki/Methylnaltrexone
“Naltrexone reverses the effects of opioids and is used primarily in the management of alcohol dependence and opioid dependence.[1]
It is marketed as its hydrochloride salt, naltrexone hydrochloride, under the trade names Revia and Depade. A once-monthly extended-release injectable formulation is marketed under the trade name Vivitrol. The closely related medication methylnaltrexone is used to treat opioid induced constipation.
Naltrexone should not be confused with naloxone nor nalorphine, which are used in emergency cases of opioid overdose.” https://en.wikipedia.org/wiki/Naltrexone