The use of placebo controls in experiments is not required to make a valid experiment. If you tell the research subjects that there are, say, 4 treatment regimens which the doctors have roughly equal expectation to work, and they’re randomly assigned to them, then there’s a lot less interference of the sort described above, and the doctors can tell the truth, and the experiment still yields useful information.
The use of placebo controls in experiments is not required to make a valid experiment. If you tell the research subjects that there are, say, 4 treatment regimens which the doctors have roughly equal expectation to work, and they’re randomly assigned to them, then there’s a lot less interference of the sort described above, and the doctors can tell the truth, and the experiment still yields useful information.