The placebo effect can work even when you know it's a placebo

The placebo effect has traditionally been explained by patient expectation — if you believe a pill will help, it sometimes does. But a landmark study by Ted Kaptchuk at Harvard (2010) gave patients with irritable bowel syndrome pills labelled 'placebo pills made of an inert substance' and told them explicitly that placebos could produce self-healing. Compared to a no-treatment group, patients taking open-label placebos reported significantly greater symptom improvement. The finding suggests the placebo response can be triggered by ritual and the therapeutic encounter itself, independent of belief.

Source: Kaptchuk et al., PLOS ONE (2010)