Merck Manual

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Pet Owner Version

Drugs Used to Treat Digestive Disorders


Patricia M. Dowling

, DVM, MSc, DACVIM, DACVCP, Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan;

Johann (Hans) F. Coetzee

, BVSc, CertCHP, PhD, DACVCP, Iowa State University

Reviewed/Revised Jul 2011 | Modified Nov 2022

A wide variety of drugs can be used to treat disorders of the digestive tract, including ones that affect the rate of movement of food through the intestines, antibiotics, drugs to suppress or induce vomiting, and drugs to treat ulcers or diarrhea.

Drugs that Affect Appetite

Appetite disorders, particularly a lack of appetite, are very common in sick animals. A veterinarian may suggest drug treatment to stimulate appetite for animals that cannot be coaxed to eat. There are several appetite-stimulating drugs; each drug works in a different way. Your veterinarian will prescribe the best drug for your pet based on how the drug can help stimulate your pet’s appetite.

Drugs that Control or Stimulate Vomiting

Vomiting is caused by conditions that stimulate the emetic center of the brain. Sometimes vomiting can be beneficial (in certain cases when a poison or toxin is eaten), but it can also be harmful in a sick or weakened animal that is likely to become dehydrated. Not all animals have the ability to vomit, so drugs that induce vomiting should never be given unless directed by a veterinarian or animal poison control specialist.

Emetic drugs are used to cause vomiting and are usually given in emergency situations after a pet has eaten a certain poison. They generally remove about 80% of the stomach contents. Syrup of ipecac is a well-known over-the-counter preparation that causes vomiting.

Antiemetic drugs are used to stop vomiting. Continual vomiting is physically exhausting and can cause dehydration, acid-base and electrolyte disturbances, and aspiration pneumonia. Antiemetic drugs are used to control excessive vomiting once a diagnosis has been made, to prevent motion sickness, and to control vomiting caused by radiation and chemotherapy.


Stomach ulcers are a common problem in animals, in association with physiologic stress, dietary management, or as a side effect of drugs that can cause ulcers. The common antacids neutralize stomach acid to form water and a neutral salt. Antacids frequently interfere with the gastrointestinal absorption of drugs that are administered at the same time. Because they require frequent dosing in dogs and cats, they are not as popular as newer therapies.

Histamine (H2)-receptor Antagonists

H2-receptor antagonists are used to treat ulcers of the gastrointestinal tract. They block stomach acid secretion by blocking H2 receptors.

Drugs Used to Treat Diarrhea

Treatment for diarrhea includes fluids, electrolyte (salt) replacement, maintenance of acid/base balance, and control of discomfort. Antiparasitic drugs or dietary treatment can also play an important role in the treatment of some types of diarrhea. Additional treatment may include intestinal protectants, motility modifiers, antibiotics, anti-inflammatory drugs, and antitoxins.

Mucosal Protectants and Adsorbents

Drugs such as kaolin-pectin formulas, activated charcoal, and bismuth subsalicylate are popular therapies for diarrhea. They work by protecting the lining of the intestines and/or absorbing the enterotoxins and endotoxins that cause some types of diarrhea.

These are all available as over-the-counter drugs, but they should not be given unless directed by your veterinarian. Some of these may change the color or consistency of feces.

Drugs that Modify Intestinal Action

Gastrointestinal motility is the rhythmic action of the intestines that moves food through the system. Anticholinergic drugs are common ingredients in antidiarrheal medications because they significantly decrease intestinal motility and secretions. They relax spasms of smooth muscles in the intestine and decrease the urgency associated with some forms of diarrhea in cats and dogs, the amount of fluid secreted into the intestine, and abdominal cramping associated with an overactive intestine. Use of anticholinergic drugs is limited in veterinary medicine because few types of diarrhea in animals are classified as overactive.

Antibiotic Drugs

The effects of antibiotic drugs in the treatment of diarrhea are unknown or unproven in most situations. However, young animals are particularly susceptible to diarrhea and inflammation of the gastrointestinal system, and they can deteriorate rapidly from diarrhea.

Nonsteroidal Anti-inflammatory Drugs (NSAIDs)

Among their many effects, nonsteroidal anti-inflammatory drugs reduce the amount of prostaglandin produced in the body. (Prostaglandin is a hormone involved in the process of muscle contraction.) These drugs may be beneficial with some types of diarrhea. However, nonsteroidal anti-inflammatory drugs should be given cautiously because they can cause adverse gastrointestinal, liver, and kidney effects.

Gastrointestinal Prokinetic Drugs

Prokinetic drugs increase the movement of food through the gastrointestinal tract. They are useful in the treatment of motility disorders because they help coordinate motility patterns. Unfortunately, some prokinetic drugs can cause serious side effects, which complicates their use.

Cathartic and Laxative Drugs

Cathartics and laxatives increase the motility (movement) of the intestine or increase the bulk of feces. These drugs are administered to increase the passage of gastrointestinal contents, to cleanse the bowel before radiography or endoscopy, to eliminate toxins from the gastrointestinal tract, and to soften feces after intestinal or anal surgery. Some cathartics work by stimulating or irritating the nerves of the intestinal lining, while others draw fluid into the intestines and increase the bulk of feces. Laxatives and fecal softeners work by increasing the water content of feces or the amount of nondigestible material in the intestines.


Anthelmintics are drugs that combat parasitic worms, many of which infest the digestive tract. The most effective anthelmintics have a broad spectrum of activity against mature and immature parasites, are easy to give, inhibit reinfection for extended periods, have a wide margin of safety, and are compatible with other compounds.

There are several classes of anthelmintics, including benzimidazoles and probenzimidazoles, salicylanilides and substituted phenols, imidazothiazoles, organophosphates, and macrocyclic lactones.


Benzimidazoles treat roundworm and flatworm infections. In horses, benzimidazoles effectively remove almost all mature strongyle roundworms, although third and fourth stage larvae are more difficult to remove. Repeated dosages are thought to be effective because the lethal effect is a slow process.

In dogs and cats, benzimidazoles are used for treatment of roundworms, hookworms, and tapeworms.


These anthelmintics are effective against gastrointestinal roundworms. In horses, pyrantel is effective against adult ascarids, large and small strongyles, pinworms, and the ileocecal tapeworm Anoplocephala perfoliata. In dogs and cats, pyrantel pamoate is effective against the common gastrointestinal worms except for whipworms. Oxantel is sometimes added to preparations of pyrantel for dogs so that whipworms are affected.


The use of organophosphates is declining. Dichlorvos is used as an anthelmintic in horses against small and large strongyles, ascarids, pinworms, and bots and in dogs and cats against roundworms, hookworms, and whipworms. Trichlorfon is used in horses against bots, ascarids, and pinworms and in dogs against roundworms, hookworms, and whipworms.

Macrocyclic Lactones

The macrocyclic lactones (avermectins and milbemycins) are products or chemical derivatives of soil microorganisms belonging to the genus Streptomyces.

Ivermectin and moxidectin are macrocyclic lactones effective against a broad range of adult and migrating larval stages of roundworm and insect parasites.

In dogs and cats, ivermectin, milbemycin oxime, moxidectin, and selamectin may be used for control of gastrointestinal roundworms.

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