Fentonium is an anticholinergic, antispasmodic and anti-ulcerogenic agent. It is quaternary analog of hyoscyamine, is a blocker of muscarinic activity and an allosteric blocker of α12βγε nicotinic receptors. It increases the spontaneous release of acetylcholine at the motor endplate without depolarizing the muscle or inhibiting cholinesterase activity. Fentonium inhibited transmitter release and depressed twitch without changing the responsiveness to noradrenaline or ATP. It is a K(+)-channel opener. Administration of fentonium bromide in rats receiving naloxone after chronic morphine treatment reduced the intensity of withdrawal signs such as increased defecation or micturition, salivation and wet-dog shakes, and elevated the nociceptive threshold values.