Morphine-N-oxide is an active opioid

Opioid

An opioid is a psychoactive chemical that works by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract...

 metabolite of morphine

Morphine

Morphine is a potent opiate analgesic medication and is considered to be the prototypical opioid. It was first isolated in 1804 by Friedrich Sertürner, first distributed by same in 1817, and first commercially sold by Merck in 1827, which at the time was a single small chemists' shop. It was more...

. Morphine itself, in trials with rats, acts 11–22 times more potent than morphine-N-oxide subcutaneously and 39–89 times more potent intraperitoneally. However pretreatment with amiphenazole

Amiphenazole

Amiphenazole is a respiratory stimulant traditionally used as an antidote for barbiturate or opiate overdose, usually in combination with bemegride, as well as poisoning from other sedative drugs and treatment of respiratory failure from other causes...

 or tacrine

Tacrine

Tacrine is a centrally acting anticholinesterase and indirect cholinergic agonist . It was the first centrally-acting cholinesterase inhibitor approved for the treatment of Alzheimer's disease, and was marketed under the trade name Cognex. Tacrine was first synthesised by Adrien Albert at the...

increases the potency of morphine-N-oxide in relation to morphine (intraperitoneally more so than in subcutaneous administration). A possible explanation is that morphine-N-oxide is rapidly inactivated in the liver and impairment of inactivation processes or enzymes increase functionality.