Why are placebos rarely used in human trials?

Answer 1

It is unethical to use placebos as every human has the right to the same chance of getting better as everyone else.

That's debatable there is recent evidence to suggest that placebos are at least as effective a treatment for "self-assessed" conditions such as depression as commercial drugs, the effect is so strong that, it works even when everybody knows it's a placebo.

But that's beside the point to answer the actual question - placebos are always used in human trials, they are a key part of the double-blind testing procedure.

1. you give everybody a random code (so the testers don't know who is who)
2. you allocate different levels of the active drug randomly to each code and an inert placebo to a control group (again the testers don't know who is getting what level of the drug)
3. regardless of the dose all the drugs look identical (so the test subjects don't know what dose they are getting and so that the people administering the drugs can't tell the testers who's getting what)

This enables the test to be carried out objectively - the testers treat everybody equally and the test subjects freely discuss they symptoms they are experiencing.