A veterinarian may recommend treatment with antihistamines for dogs with mast cell tumors. Mast cells contain many types of substances that have different functions within the body, one of them being histamine. The use of anti-histamines is recommended to prevent side effects that release of histamine from the mast cells may have on the pet. Anti-histamines do not have an anti-tumor effect. In other words, anti-histamines do not get rid of mast cell tumor disease in the animal. There are two types of histamine receptors in the body, type 1 (H1) and type 2 (H2) histamine receptors.
Use of type 1 anti-histamines for mast cell tumors in dogs
Type 1 histamine receptors (H1) are involved in allergic reactions that result in sudden swelling and itching. Owners often report that a mast cell tumor appears to increase and decrease in size over time. That behavior is likely due to the release of histamine from mast cells stimulating H1 receptors and causing edema and swelling of the tissues. This effect may subside over time after there is no further release of histamine from the mast cells making it appear as if the mass is shrinking in size.
Type 1 anti-histamines are often used the day of surgery to prevent a type 1 histamine reaction while surgery is being performed. They may also be used prior to surgery to decrease any pre-existing swelling of a tumor prior to removal. In circumstances where the mast cell tumor disease is systemic (involves multiple sites), type 1 anti-histamines may be recommended with chemotherapy as part of treatment for the animal to help avoid histamine related side effects from the disease.
There are a number of over-the-counter drugs which block H1 receptors, such as diphenhydramine (e.g. Benadryl) and chlorpheniramine. It is important to understand that use of H1 in dogs with mast cell tumor has no anti-tumor activity. There use only blocks potential side effects that might occur in dogs with mast cell tumor disease.
Use of type 2 anti-histamines for mast cell tumors in dogs
Type 2 histamine receptors (H2) are found in the stomach. Stimulation of H2 receptors on the stomach lead to increased acid secretion. A sign that a dog may be affected by histamine related increased acid secretion include decreased appetite, vomiting, diarrhea or black tarry stools, which is an advanced sign of upper intestinal bleeding due to gastrointestinal ulcers.
Pepcid is a common over-the-counter H2 receptor blocker antacid medication used in dogs. Although they do not block H2 receptors, proton-pump inhibitors are considered by many to be a more potent anti-acid drug than H2 blockers. They block the final step in acid secretion that is stimulated by H2 receptor activation. A common over-the-counter proton pump inhibitor is omeprazole (Prilosec). Antacid medication is sometime administered the day of surgery or when systemic disease exists to help avoid histamine related increased acid production from the disease. It is important to understand that use of H2 receptor blockers or proton-pump inhibitors in dogs with mast cell tumor has no anti-tumor activity.