By the same token, giving someone caffeine and telling them it's a sleep aid should work. Or giving them a sedative and telling them it will keep them awake. Or telling them that raping a virgin will cure AIDS. Okay, that last one backfired: sub-Saharan Africa didn’t trade its status as the AIDS capital of the world for a massive increase in infant and child rape—it made its AIDS epidemic even worse by adding infant and child rape to it. I digress.
Since placebo is the mind creating a particular reality for itself, giving someone poison and telling them it will make them immortal could theoretically work if the person believed it would...right? No, that’s been debunked too—many Chinese Emperors died from the arsenic and mercury in an "Elixir of Life" they believed would confer immortality.
Okay, so there are limits to what the placebo effect can achieve. But, if you gave someone a placebo and told them it was twice as effective as x instead of just telling them it was x, might it have a proportionately greater effect? If so, why stop at twice? Tell them it’s 10x as effective, or up to 100x as effective. Tell them it will increase HGH production. Tell them it will make them telepathic. Maybe placebo is the gateway to superpowers!
I mean, it sounds like a crock of shit, but lots of things are a crock of shit and still work simply because people believe they will! Why not take it a step further?