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Effects of diphenhydramine (Benadryl) in dogs with mast cell tumours undergoing surgery at OVC

** Please note this study is closed/completed and no longer recruiting patients ** 


To determine the validity of using pre-emptive diphenhydramine to prevent responses to histamine release in dogs with mast cell tumours.


Mast cell tumours (MCT) are the most frequent malignant skin tumour in dogs. MCT release histamine, which potentially causes vasodilation and hypotension. Diphenhydramine (DPH; also known as Benadryl®) is an antihistamine recommended by some clinicians for patients undergoing surgical excision of MCT, with the theory that it can prevent the potential effects of histamine released during surgical manipulation of the tumour. The benefit of DPH administration for this purpose has not been proven and routine pre-operative treatment with DPH is not always implemented and outcomes are positive despite its lack of administration.

Study Design:

Patients will be randomly assigned to receive DPH or placebo (saline) prior to MCT excision.  Normal anesthetic and surgical protocols for MCT removal and standard of care for clinical cases will be applied.  Qualified anesthesia personnel will carry out anesthesia and provide extensive monitoring of cardiorespiratory parameters throughout the procedure so that rescue measures can be instituted if patients are deemed to benefit from DPH administration at any time during surgery.  Histamine and DPH levels will be measured in blood drawn from a previously placed catheter.

Novelty: This study will determine if pre-emptive DPH administration is beneficial to patients undergoing excision of MCT.


  • Extensive cardiac monitoring during surgery (no cost to owner).

Samples required:

  • 6 blood samples (~36ml in total).

Inclusion Criteria:

  • Confirmed Mast Cell Tumour
  • Surgical excision to occur at OVC
  • Ideally greater than 8kg body weight

Exclusion Criteria:

  • Cardiac or renal disease
  • Diphenhydramine (Benadryl) administered less than 12 hours before surgery


  • Dr. Alex Valverde (PI)
  • Dr. Andrea Sanchez


Dr. Andrea Sanchez (Investigator), Cell #: 226-924-5920

Vicky Sabine (PhD), Clinical Research Coordinator, OVC
Email:; Work Cell #: 226-218-0338

Funded by OVC Pet Trust.

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