Available in 10, 20, and 40 mg tablets, and injectable

Brand Names: Pepcid, Pepcid AC, Pepcid RPD

Background

Stomach ulceration in humans is a prominent medical condition, and there has long been pressure to develop effective and convenient ways to control it. Until relatively recently, we relied on simply neutralizing stomach acid by pouring alkaline solutions (i.e., Alka Seltzer, Tums, Rolaids, etc.) into the stomach. In fact, ulceration is a complicated process and not so easily solved.

Control of stomach acidity is a very important factor in the treatment of stomach ulcers. The acid we need to digest food contributes to the creation of ulcers and perpetuates them. Stomach acid secretion is controlled mainly by three biochemicals: gastrin (a hormone secreted when food is perceived in the stomach), acetylcholine (a neurotransmitter), and histamine (the same substance responsible for the unpleasant effects of allergy). Any of these substances will instruct the glands of the stomach to release acid, which we would like to prevent if ulcers are in question.

Famotidine is a special antihistamine, as are its cousins cimetidine (Tagamet HB) and ranitidine (Zantac). This class of antihistamines is not useful in combating familiar allergic symptoms (itching, sneezing, stuffy nose, etc.) In allergy, histamine causes unpleasant effects by binding so-called H1 receptors. Famotidine, ranitidine, and cimetidine instead bind to histamine receptors in the stomach called H2 receptors.

Cimetidine was the first such H2 blocker available, and each generation has brought about improvements in terms of fewer drug interactions and stronger effects. Famotidine is the longest-lasting of the H2 blockers (usually, one dose lasts 12-24 hours). Famotidine is 32 times stronger in its ability to inhibit stomach acid than cimetidine and is nine times stronger than ranitidine. A newer H2 blocker called nizatidine is now available that offers the additional advantages of especially rapid onset of action and some effect on normalizing stomach contractions as well.

Famotidine is currently available in an over-the-counter formulation, making it highly convenient for pet owners to obtain (though obviously, you should not consider using medications licensed for human consumption without specific instructions from your pet's veterinarian). Famotidine is especially useful for pets with acute vomiting, though as technology has advanced, H2 receptor blockers are gradually becoming supplanted by proton pump inhibitors (such as omeprazole), which are even stronger antacids. Famotidine has reduced effectiveness after about two weeks of use and is best used for conditions expected to resolve within that time frame.

How this Medication is Used

Famotidine is useful in any situation where stomach irritation is an issue and ulceration is a concern. It is often used in the treatment of canine parvovirus, after ingestion of a toxin that could be ulcerating (overdose of aspirin, for example), any disease involving protracted vomiting, or in combination with other medications that may have stomach irritating properties so as to mitigate those properties.

In diseases involving frequent vomiting or regurgitation, the esophagus (tube connecting the mouth and stomach) can be ulcerated by continuing exposure to vomit/stomach acid. Antacids are also helpful in this type of situation to reduce damage to the esophagus. Megaesophagus would be a condition where a long-acting antacid such as famotidine could be helpful in mitigating injury to the esophagus; however, there is a trade-off in protection against aspiration pneumonia, in that stomach acid hampers bacterial growth should stomach contents enter the lung.

Famotidine is directly helpful in managing nausea in species where there are H1 receptors in the brain's chemoreceptor trigger zone (an area involved in stimulating vomiting). In other words, famotidine is not only an antacid but also an antinauseal for dogs, but for cats is only an antacid.

Famotidine is given once or twice daily. If a dose is accidentally skipped, simply give it when you remember.  Do not double up on the next dose, but the broad safety range of this medication allows for leeway in the spacing of doses. 

Famotidine works best when given before the first meal of the day but can be given with or without food.

Side Effects

The H2 blockers as a group have a limited potential for side effects, hence their recent release to over-the-counter status.

Interactions with Other Drugs

Antibiotics of the cephalosporin class (particularly cefpodoxime) may not work as well when given with famotidine. It is best to separate their administration by at least 2 hours.

There are some drugs that are absorbed better in the presence of stomach acid (examples: itraconazole, ketoconazole, fluconazole, and oral iron supplements). These drugs should be given at least an hour apart from famotidine.

Cefpodoxime does not absorb as well in the presence of famotidine. This effect is reduced by giving both medications with food.

Oral iron supplements do not absorb into the body as well in the presence of famotidine. Stagger their administration by at least an hour.

Concerns and Cautions

It appears that famotidine is safe for use in pregnancy.

Famotidine tablets should be stored at room temperature and protected from light.