H₂ inhibitors and/or antagonists act by competing with histamine for histamine type-2 receptor sites on the parietal cell and therefore blocking histamine-stimulated gastric acid secretion. The two most popular H₂ inhibitors and/or antagonists used in horses are cimetidine, marketed as Tagamet® and ranitidine, marketed as Zantac®. H₂ inhibitors/antagonists can be used with antacids like Nutrient Buffer®, but is likely not needed. Nutrient Buffer®, uses time-release ingredients; it is effective and reliable all by itself. Nutrient Buffer® is a natural prostaglandin analogue (similar to lots of pasture for your horse).
Drugs have many drawbacks, and H₂ blockers have some unique ones. Histamine H₂ blockers do exactly what their name implies, they block the histamine from telling the acid producing cells to do their job. The result is another very effective acid blocking drug. When your horse builds a tolerance after prolonged daily use, you find yourself continually increasing the dosage to achieve the same results.
These side effects are very accurately described in the PDR/Physician’s Desk Reference for humans, as these drugs have been used for humans much longer than for horses.
Other side effects reported are: headaches, abdominal discomfort and pain, agitation, anemia, changes in liver function, constipation, depression, diarrhea, difficulty sleeping, dizziness, hair loss, hallucinations, heart block, hepatitis, inflammation of pancreas, involuntary movements, irregular heartbeat, jaundice, joint pain, muscle pain, nausea and vomiting, rapid heartbeat, rash, reduced white blood cells, reversible mental confusion, severe allergic reactions, sleepiness, slow heartbeat, swollen face and throat and vertigo.
The driving factors that lead to a horse suffering from excessive digestive acidity in the first place are not addressed when solely using these drugs. Re-evaluate your feed program, and water quality, if you are considering use of H₂ blockers, inhibitors or antagonists drugs.