Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




(for veterinary information only)







The term “intestinal motility” refers to the ability of intestinal muscle to contract and move intestinal contents from the mouth end of the animal to the anal end of the animal.

In older times, it was thought that diarrhea was the result of excessive motility in the intestinal tract, that intestinal contents were rushed through prematurely in a sequence of muscular spasms. For years, diarrhea was symptomatically treated with medications designed to abolish intestinal muscle tone with the idea that these spasms could be prevented. Results were disappointing.

More recently, the details of intestinal contractions have been elucidated. There are many types of muscle contractions in the intestine. There is the forward-propelling “peristaltic” contraction that moves intestinal contents forward. There is “segmental” contraction which can divide the intestine into small segments where different materials can be sequestered temporarily for absorption and digestion. There is also an overall general muscle tone or “tonus” to the intestinal tract and it is this “tonus” that controls the speed by which intestinal contents moves. More muscle tone/tonus means slower movement of intestinal contents, more absorption of water from the intestinal contents, and less tendency towards diarrhea. When this muscle tone is abolished, intestinal contents simply pour through the tract and diarrhea is worse.



Despite its inability to produce recreational euphoria, loperamide is actually a member of the opiate class of drugs. Opiates have numerous effects that have made them beneficial (as well targets of abuse) for centuries; one such beneficial effect is an increase in general muscle tone of the small intestine. As described above, increasing tonus means more absorption of water and nutrients and less diarrhea. Loperamide was developed to exploit this positive effect without other opiate properties (addictiveness, euphoria, sedation etc.).

An additional benefit of loperamide could be helpful in dogs with fecal incontinence. Loperamide may have a tightening effect on the internal anal sphincter, which could be helpful in dogs with fecal incontinence issues.

Loperamide may be given with or without food. If a dose is accidentally skipped, do not double up on the next dose but give the dose when it is remembered and time the subsequent dose accordingly.




Side effects are not common with loperamide but constipation and/or bloating are possibilities. Since loperamide is an opiate, tranquilization is a possible side effect. These are generally minor issues only and resolve with discontinuation of loperamide.

Serious side effects are rare. Pancreatitis is a reported rare but potential side effect and gas distension has the potential to progress to bloat. Again, these potential side effects are rare but it is important to report them as possibilities.




Opiates should not be given to patients concurrently taking L-Deprenyl (Anipryl) or other Monoamine Oxidase Inhibitors (such as amitraz-containing tick control products). Actually serious reactions have only been observed between L-Deprenyl and mepiridine (demerol) but, to be safe, the warning has been extended to all opiates.

Tranquilizers and antihistamines should not be used with opiates as sedating properties may be overly enhanced.

The following drugs may increase the activity/blood levels of loperamide and increase the potential for side effects:

  • Amiodarone: A heart medication
  • Carvedilol: A heart medication
  • Cyclosporine: An immuno-modulator
  • Erythromycin: An antibiotic
  • Imipramine: A behavioral medicine
  • Itraconazole, Ketoconazole: Antifungal medications
  • Propranolol: A heart medication
  • Quinidine: A heart medication
  • Trimethoprim sulfa: An antibiotic
  • Verapamil: A heart medication



Use of this medication may falsely elevate lab tests for pancreatitis (amylase and lipase levels).

This medication is contraindicated for diarrheas involving intestinal toxins (parvovirus enteritis or patients with liver failure would be a good examples). In these cases, the absorption enhancing effects of loperamide could be a serious problem as one would not want to enhance the absorption of intestinal toxins.

This medication is on the list of drugs influenced by a mutation called the "MDR-1" mutation (recently renamed the "ABCB1" mutation). This mutation is common in Collies and Collie-type breeds. There is a protein called the P-glycoprotein and it is involved in keeping certain drugs out of certain body tissues (the central nervous system in particularly). Many Collies have a mutation in the gene that produces this protein making their central nervous system more available to certain drugs than would be the cases in dogs with normal P-glycoproteins. Loperamide is one of the drugs that can be inappropriately transported into the central nervous system in high levels if there is inadequate P-glycoprotein to prevent it, making dogs with this mutation extra sensitive to sedation side effects. It is best to avoid this medication or use it cautiously in Collies and related breeds such as Australian Shepherds, Old English Sheepdogs, and Shetland Sheepdogs. Not all collies/collie-types have this mutation and it is possible to have a dog tested. Any white-footed member of a herding breed should be considered for such testing. For more information click here.


Collie head mf



Australian Shepherd head




Loperamide should not be used in debilitated patients. Opiates can be problematic for dogs with Addison's disease, hypothyroidism, or increased pressure in the skull (brain tumor, hydrocephalus, etc.)


Short version (to help us comply with "Lizzie's Law")

 Page last reviewed: 3/17/2022