For the most part, it's good advice to tell people to follow their dreams. In our darker moments, a lot of dreams can seem impossible to attain and some helpful encouragement from someone who knows what we're capable of can make the difference between achieving it and abandoning it.
But, as sad as it is to say, it can be outright irresponsible to encourage people towards certain dreams. For instance, if someone dreamed of being a breatharian who only subsists on air and sunlight, encouraging them is, as The Guardian reported, the same as sentencing them to die of dehydration and starvation.
Although what we're talking about today isn't so obviously lethal as that, it still involves the use of dangerous substances for questionable reasons.