OVC researchers are investigating whether a common antihistamine used to treat allergies and other conditions is also helpful when given to dogs undergoing mast cell tumour (MCT) surgery.  One of the most common types of malignant skin cancer in dogs and cats, MCT release histamine, a compound produced by most cells in response to injury and in allergic and inflammatory reactions.  The release of histamine causes the blood vessels to widen and lowers blood pressure (hypotension).  This natural defence mechanism may put dogs at risk during surgery.  Some clinicians prescribe the use of a histamine blocker called diphenhydramine — the active ingredient in the over-the-counter medication Benadryl — before surgery to minimize the chances of cardiovascular problems.  However, the benefits of using diphenhydramine in this way have not been scientifically proven, and until a recent study by this OVC research team, the complex interactions between the drug and the canine body — and the data needed to safely and effectively administer the drug — were not fully understood.  This follow-up study will determine whether diphenhydramine has a beneficial effect in preventing hypotension in dogs undergoing MCT surgery.  It will also measure and compare histamine concentrations in the blood of MCT patients with that of healthy dogs.