The Unquantified Placebo Effect

Upon viewing the Headline - No Standard for the Placebo? I had hoped for something more in line with my current interests. Alas no, the news item relates to how the placebo pill contents are rarely published and may have important bearing on experimental results. I was off on a completely different tangent, wondering about the variability of the placebo response and the implications this has for interpreting clinical trial results. Turns out it is rather variable.




The placebo effect is certainly not a constant. For example, is the size of the placebo effect altered when the clinicians conducting the trial are of a different race than the participants in the trial? Is the size of the placebo effect across different pathologies or categories of pathologies? For example, would more people be inclined to recover from depression even though they are only receiving a placebo as opposed to people with high blood pressure and receiving a placebo? With the increasing use of third world countries for clinical trial purposes what implications does this have for extrapolating the findings of these clinical trials to people living in the First World? Am I the only person asking these questions? Certainly not, there must be others, so let us see ... .

A simple string in medline ... "placebo effect" AND variation AND pathology ... generated 4 hits. The below is a good example of easily the placebo effect can impact on clinical trial results.

Moderators of placebo response to antipsychotic treatment in patients with schizophrenia: a meta-regression.
Psychopharmacology (Berl). 2003 Feb;166(1):1-10. Epub 2002 Dec 20.

CONCLUSIONS: Placebo-controlled trials of short duration (6-8 weeks) are vulnerable to substantial placebo response. Recruiting patients with more severe pathology to mitigate placebo response does not appear to offer benefits and may even be counterproductive. Meta-analyses based on individual patient data offer the potential for much more detailed and inferentially sound exploration of factors affecting placebo response and are highly recommended.

And this ... 


[How can an effective drug to treat irritable bowel syndrome be successfully developed?]
Gastroenterol Clin Biol. 2009 Feb;33 Suppl 1:S26-34.

RESULTS: Developing IBS drugs remains a major challenge, as numerous factors, related or unrelated to the nature of the disease itself, interfere with the demonstration of efficacy : the multiplicity of physiopathological mechanisms, wide variation in symptoms across patients and over time, associated psychological traits and environmental aspects, and a very significant placebo effect.

So there you go, a 30 second search and I find two articles highlighting the variability of the placebo response. Within the confines of the study itself the effect is largely without relevance but the very point of clinical trials is  the presumption that the placebo effect will hold constant across the population. It doesn't. The "gold standard" of medical research is devalued.