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Loperamide (Imodium AD)
Revised: November 29, 2023
Published: January 01, 2001

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Imodium AD

Available in 2 mg caplets and as oral liquid


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.

Historically, it was thought that diarrhea was the result of excessive motility in the intestinal tract in 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. The 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 that 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 move. 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.

How this Medication Works

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 as 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 absorbing more 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.

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

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 are resolved when the pet stops taking it. 

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.

Dogs with the MDR1 mutation may experience neurologic incoordination, dilated pupils, sedation, and drooling (see below).

Interactions with Other Drugs

Opiates should not be given to patients concurrently taking L-Deprenyl (Anipryl) or other Monoamine Oxidase Inhibitors. Actually, serious reactions have only been observed between L-Deprenyl and meperidine (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:

Concerns and Cautions

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

This medication should not be used for diarrhea involving intestinal toxins (parvovirus enteritis or patients with liver failure would be 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.

The MDR1 Mutation

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 particular). 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 at 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. 

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.)

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