The value of EBVM and a move forward

Veterinarians need to closely inspect information, as well as patients, for an evidence-based diagnosis

Veterinarians need to closely inspect information, as well as patients, for an evidence-based diagnosis.
Veterinarians need to closely inspect information, as well as patients, for an evidence-based diagnosis.

One of the key elements of evidence-based veterinary medicine (EBVM) is the need to regularly review and update our understanding and practices as new evidence is developed. EBVM is an ongoing process that supports continuous learning and improvement in veterinary care. Of course, the most difficult aspect of this is when new evidence emerges showing an established belief or practice is mistaken or ineffective. Cognitive dissonance is the formal term for the painful experience of encountering evidence that undermines a cherished belief, and it is a powerful impediment to change.1

Being human

Humans naturally struggle to give up our beliefs, and the more entrenched or longstanding a belief, the more integrated it is into our general world view, the harder it is to abandon this belief despite strong evidence against it.

Truly dramatic shifts in perspective and practice sometimes require the replacement of one generation of practitioners with another not already committed to existing views and practices. Even unquestionably ineffective practices, such as homeopathy, or interventions that do more harm than good, such as bloodletting, can persist and be vigorously defended long past the point where the evidence clearly shows their worthlessness because of the natural human reluctance to revisit and revise our beliefs.1-4

The bad news is none of us are immune to this. I have spent a great deal of my career studying EBVM and the methods for mitigating the influence of cognitive bias on clinical decision-making, and, yet, I have been a victim of many of the errors I try to teach others how to avoid. My favorite example of this is the nocebo effect.

Two sides of a coin

The nocebo effect is the evil twin of the placebo effect. Our expectations and features of how healthcare is delivered can cause patients, or veterinary clients, to perceive improvement in health or benefits to treatments when there are none. This is one important element of what we call the placebo effect. However, expectations and aspects of the clinical encounter can also cause us to perceive negative effects of treatment or deterioration in our symptoms when these also are not really present.

In my case, I started a preventative medication I knew a bit about, and I experienced a mild side-effect patients commonly report with this drug. After multiple rounds of starting and stopping the medication, and trying different doses and different formulations, I asked my doctor to conduct an experiment. I asked him to give me the real drug for a period of time and then a placebo and not tell me which was which so I could see if my symptoms were real or nocebo effects. Unfortunately, my doctor was not comfortable with this plan, and I eventually stopped the treatment.

Years later, as I got older, the potential risks of not taking this therapy grew, and eventually I decided, along with my new doctor, I probably should be taking it even if I had to tolerate the mild side-effects. Miraculously, I have now been on this treatment for several years without a hint of the problems I previously experienced. What is more, in the intervening time, clinical trials have shown the common side-effects are only reported when patients know they are taking the drug, and these do not occur in blinded clinical studies. Even knowing I might be experiencing a nocebo effect did not prevent it or save me from the misguided decision to stop a potentially valuable treatment based on illusory symptoms.

We all do it

My point, which I try to make as often as I can, is none of us are immune to the cognitive biases and errors that mislead us. We all believe our personal experience to be probative even when the evidence is overwhelming that anecdote and uncontrolled observations are highly misleading. We do not have false beliefs or persist in ineffective practices because we are stupid or venal or brainwashed by malign actors like “big pharma.” We make these mistakes because we are human, and these are intrinsic, inextricable features of how our brains work.

That’s the bad news. The good news is science generally, and EBVM in particular, can help. As is often said in 12-step addiction recovery programs, we can only begin to solve our problems once we admit they exist.

If we acknowledge and truly understand our limitations as individuals, we can begin to make use of the powerful tools of science and EBVM, tools built, improved, and maintained over generations by a community to help us as individuals make better decisions for our patients.

On a practical level, this means deciding how confident we should be in each belief or practice we have, and proportioning this confidence to the strength of the scientific evidence (not our personal experiences, unless this is the only evidence there is). In these columns, I have tried to illustrate this process, often by challenging established beliefs or giving examples of widespread practices shown to be ineffective by good research evidence despite the misleading support of anecdotal experience.

Tramadol probably doesn’t effectively treat pain in dogs, and our enthusiasm for it has turned out to be misguided.5 Lysine and metronidazole probably don’t have much benefit, despite widespread and longstanding use.6,7 And many less plausible treatments, from acupuncture to homeopathy, are even less likely to actually provide the benefits their adherents claim.8,9

The number of people who believe in something, and the length of time people have believed in it, are poor indicators of whether or not that belief is true. Scientific research is itself flawed and sometimes misleading, but it is a lot more likely to get us to the truth than belief founded on experience, anecdote, theory, and tradition.

Of course, scientific evidence doesn’t have to be exclusively negative. Cannabis turns out to have some real potential medical benefits, which research has sifted, like wheat from the chaff, out of the innumerable anecdotal claims made for it.10,11 And despite the fact the value of gabapentin as a sedative for cats coming into the clinic seems “obvious,” it is comforting to have actual objective evidence to support this belief, given how many equally “obvious” beliefs have failed us.12

The focus of these columns, and much of EBVM, on challenging poorly founded beliefs does not negate the value of testing that confirms our intuitions and practices. The greatest benefit of science often lies in showing us our faults so we can do better, but science also tells us when we’ve gotten it right, and that is just as important.

The core message of EBVM, and of my writing, is all of us struggle with universal human limitations, whatever our personal intelligence, talent, education, and experience. True humility is recognizing this and taking advantage of the tools available to form the most accurate understanding of nature and the most effective set of clinical practices we can to best serve our patients and their owners. EBVM is one of those tools, and it is most useful to us if we understand our need for it.

The work of EBVM, for each of us as individual practitioners and for the veterinary community as a whole, is never done. For all the columns I have written in the last five years, as well as all of my efforts elsewhere to use scientific evidence to inform my own clinical practices and those of others, I have only touched on a tiny fraction of the questions we need answers to and the evidence out there to help us answer them. I hope these efforts have been useful and informative.

A farewell

Sadly, the time has come for me to hand over this column to others. I have been honored to have this opportunity to share my passion for EBVM and my own process of learning and discover with the readers of Veterinary Practice News. Though I will always stay engaged with EBVM, I have taken up some new topics and other platforms for contributing to the veterinary profession.

I offer a heartfelt thank you to Dr. Narda Robinson, the previous author of this column, for suggesting I take it on, to my editors for their support and guidance, and to the readers for making the whole exercise worthwhile. I look forward to following this column as a reader and learning from the insights and perspective of those who carry it forward.

Brennen McKenzie, MA, MSc, VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.


  1. McKenzie BA. Veterinary clinical decision-making: cognitive biases, external constraints, and strategies for improvement. J Am Vet Med Assoc. 2014;244(3):271-276. doi:10.2460/javma.244.3.271
  2. McKenzie B. Placebos for Pets: The Truth About Alternative Medicine for Animals. Ockham Publishing; 2019.
  3. Greenstone G. The history of bloodletting. BCMJ. 2010;52(1):12-14. Accessed July 22, 2022.
  4. Vite CH, Head E. Aging in the Canine and Feline Brain. Vet Clin Small Anim Pract. 2014;44(6):1113-1129. doi:10.1016/J.CVSM.2014.07.008
  5. McKenzie B. Is tramadol an effective analgesic for dogs and cats? Vet Pract News. June 2018:32-33.
  6. McKenzie B. Lysine: A therapeutic zombie? Vet Pract News. May 2018:26-28.
  7. McKenzie BA. Think metronidazole works? Maybe not so much. Vet Pract News. November 2019:26-27.
  8. McKenzie B. Acupuncture-Why it is a sticking point among today’s veterinary professionals. Vet Pract News. October 2019:40-41.
  9. McKenzie B. Veterinary homeopathy: Why are we still talking about this? Vet Pract News. December 2019:30.
  10. McKenzie B. A conclusion on cannabis? Vet Pract News. July 2019:26-27.
  11. Stockings E, Zagic D, Campbell G, et al. Evidence for cannabis and cannabinoids for epilepsy: a systematic review of controlled and observational evidence. J Neurol Neurosurg Psychiatry. 2018;89(7):741-753. doi:10.1136/jnnp-2017-317168
  12. Gurney M, Gower L. Randomised clinical trial evaluating the effect of a single preappointment dose of gabapentin on signs of stress in hyperthyroid cats. J Feline Med Surg. 2022;24(6):e85-e89. doi:10.1177/1098612X221091736

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