Difenoxin is a 4-phenylpiperidine which is closely related to the opioid analgesic meperidine. Difenoxin alone is a USA Schedule I controlled drug, as it may be habit forming. However, it is listed as a Schedule IV controlled drug if combined with atropine, which is added to decrease deliberate misuse. Motofen(R) is a brand mixture which combines atropine sulfate and difenoxin hydrochloride. It is approved by the FDA to treat acute and chronic diarrhea. Difenoxin is an active metabolite of the anti-diarrheal drug, diphenoxylate, which is also used in combination with atropine in the brand mixture Lomotil(R). It works mostly in the periphery and activates opioid receptors in the intestine rather than the central nervous system (CNS). Difenoxin is also closely related to loperamide, but unlike loperamide it is still capable of crossing the blood brain barrier to produce weak sedative and analgesic effects. However, the antidiarrheal potency of difenoxin is much greater than its CNS effects, which makes it an attractive alternative to other opioids. Motofen(R) is a combination of atropine, an anticholinergic drug, and difenoxin, an antidiarrheal drug. It has been used in many countries for many years as a second line opioid-agonist antidiarrheal, which exists an intermediate between loperamide and paragoric. Diarrhea which is a result of cyclic or diarrhea predominant Inflammatory Bowel Syndrome may not be treated effectively with difenoxin, diphenoxylate, or loperamide. As such, diarrhea and cramping which does not respond to non-centrally acting derivatives or belladonna derivatives such as atropine are often treated with conservative doses of codeine. In patients with acute ulcerative colitis, as induction of toxic megacolon is possible, and thus use of Motofen(R) is cautioned. Motofen(R) has been assigned pregnancy category C by the FDA, and is to be used only when the potential benefits outweigh the potential risk to the fetus. The safety of use during lactation is unknown and thus not recommended. Each five-sided dye free MOTOFEN® tablet contains: 1 mg of difenoxin (equivalent to 1.09 mg of difenoxin hydrochloride) and 0.025 mg of atropine sulfate (equivalent to 0.01 mg of atropine). Difenoxin acts as an antidiarrheal by activating peripheral opioid receptors in the small intestine and thereby inhibiting peristalsis. However, research has suggested that non-opioid receptor pathways exist. This would explain the potent antidiarrheal effects of difenoxin despite only limited opioid action.

Atropine inhibits the muscarinic actions of acetylcholine on structures innervated by postganglionic cholinergic nerves, and on smooth muscles which respond to endogenous acetylcholine but are not so innervated. As with other antimuscarinic agents, the major action of atropine is a competitive or surmountable antagonism which can be overcome by increasing the concentration of acetylcholine at receptor sites of the effector organ (e.g., by using anticholinesterase agents which inhibit the enzymatic destruction of acetylcholine). The receptors antagonized by atropine are the peripheral structures that are stimulated or inhibited by muscarine (i.e., exocrine glands and smooth and cardiac muscle). Responses to postganglionic cholinergic nerve stimulation also may be inhibited by atropine but this occurs less readily than with responses to injected (exogenous) choline esters. Atropine is relatively selective for muscarinic receptors. Its potency at nicotinic receptors is much lower, and actions at non-muscarinic receptors are generally undetectable clinically. Atropine does not distinguish among the M1, M2, and M3 subgroups of muscarinic receptors.

Atropine inhibits the muscarinic actions of acetylcholine on structures innervated by postganglionic cholinergic nerves, and on smooth muscles which respond to endogenous acetylcholine but are not so innervated. As with other antimuscarinic agents, the major action of atropine is a competitive or surmountable antagonism which can be overcome by increasing the concentration of acetylcholine at receptor sites of the effector organ (e.g., by using anticholinesterase agents which inhibit the enzymatic destruction of acetylcholine). The receptors antagonized by atropine are the peripheral structures that are stimulated or inhibited by muscarine (i.e., exocrine glands and smooth and cardiac muscle). Responses to postganglionic cholinergic nerve stimulation also may be inhibited by atropine but this occurs less readily than with responses to injected (exogenous) choline esters. Atropine is relatively selective for muscarinic receptors. Its potency at nicotinic receptors is much lower, and actions at non-muscarinic receptors are generally undetectable clinically. Atropine does not distinguish among the M1, M2, and M3 subgroups of muscarinic receptors.