Homeopathy

And unethical.

Not really.

For self-limiting, non-threatening conditions, placebos are very useful. They make people feel better. And they're cheap.

And unethical.

You could argue placebos can be used unethically, but a harmless sugar pill cannot in itself be unethical. It's like saying sticking plasters are unethical.

Belief in the efficacy of homeopathy has built up over a long time, increasing its placebo effect. The more you believe it works, the more benefit you will feel. Up to a point, I'd be happy to be duped if it made me feel better.