Placebos work, so why shouldn't GPs be allowed to prescribe them?
This is not an uncommon argument, even among physicians. But there's a simple rebuttal, in my view: Giving a placebo conflicts with the patient's right to be informed.
Patients deserve accurate and unbiased information about the risks and benefits of every medicine they are offered. Indeed, at every clinic I've worked in, the patient signs a form stating "I have been informed about the risks and benefits of this medication" (or words to that effect). If I give a patient a treatment that I know for certain is useless- let's say, a sugar pill that is dummied up to look like a prescription medication-- then I would have to inform the patient that I know the treatment to be useless. It's not OK for me to withhold that information, or to keep secrets from the patient, even if I think it's "for their own good".
(There's one exception to this rule: If the patient is enrolled in a clinical trial, they can be randomly assigned to receive either active treatment or placebo. But that's a special case, with special rules. And even in those cases the patient must be told about the process of randomization and how it works).
It *is* OK to give treatments when the evidence that it works is weak, or dubious, or where we just don't know if it is an effective treatment or not. Sometimes, when the risk of the treatment is minimal, that can be a reasonable thing to do (I'm thinking of, for example, folate supplementation for clinical depression). But you have to be honest with the patient about what you're doing.