Dr. Alex Valverde is 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, MCTs 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. This study will determine whether a histamine blocker called diphenhydramine — the active ingredient in the over-the-counter medication Benadryl — before surgery effectively and safely minimizes the chances of cardiovascular problems.