Somebody mentioned that if a effect produced from a action was placebo derived, that over time the effect will lose it's magnitude to the point it becomes non-existent; I can't find anything to verify this one way or another, does anybody know if it's true?

I'd imagine one would become conditioned by the placebo eventually. Seems like that the placebo would constantly reinforce the conditioning, creating a permanent effect inside a vacuum.

I think they would become conditioned if they took the placebo in every instance, then naturally by habit there belief would be enforced. Although if they had higher expectations for there placebo, and or they are constantly forced to second guess its effects with perhaps a dash of consequence, I think its effects would then diminish.

Basically IMO I believe a large reason for the placebo or any belief for that matter is It has perceived benefits/its convenient. Take religion for example. When those benefits become an inconvenience I would bet the belief would likely fade.

For example I think having strong value for the truth may be a helpful antidote, if the devil ain't to tricky.

This is definitely not true in some cases. There are all kinds of BS homeopathic and neutraceutical products out there that do nothing and yet people take them day after day convinced that they help.

For example, products that claim they prevent you from getting a cold, or even if a cold breaks through it won't be as bad or last as long. As colds are variable and self-limited, every day is a confirmation of the placebo, literally no matter what happens.