[refurb]

Yes, it is possible to compare two placebo controlled trials and get a rough estimate of the relative efficacy of the two drugs. Many of the prescribing decisions that physicians make are based on these indirect comparisons.

However, it's only a rough estimate. Even if the patient populations are nearly identical, you can often see different outcomes.

To give you an example: Crohn's disease is an autoimmune disease of the large (and sometimes small) intestine. If you look at the clinical trials for the biologics used to treat the condition, you'll see remarkably varied outcomes, even in the placebo arm.

In other words, if drug A show 50% vs. 20% efficacy vs. placebo and drug B showed 70% vs. 40% efficacy vs. placebo, which is the more efficacious drug?

[roc]

> "drug A show 50% vs. 20% efficacy vs. placebo and drug B showed 70% vs. 40% efficacy vs. placebo"

Which raises a further (or more basic) confounding factor: the placebo effect isn't fixed.

It does vary between trials and even appears to be steadily increasing in potency over time. [1]

So first-to-market drugs have an added advantage when naively considering "improvement vs placebo" -- as their test were run years ago, when the placebo effect itself was a weaker opponent.

[1] http://www.wired.com/medtech/drugs/magazine/17-09/ff_placebo...